We use a company called Joiner systems .com in Toronto, but there are many that do this. They provide everything you need as far as wall panels, ceilings and all the trim necessary to make a clean fit up. The ten boxes I just had refitted are a mix of 32 and 34 foot boxes with one 38 in the mix.
In my opinion ISO boxes are great, but he biggest drawback is refitting. Due to their incredibly low cost of as low as 2k for a solid box they are tempting to fit. The drawback is that the interior of an ISO box and standard dimensioned anything in the construction trades dont mix. Every sheet of wall panel and stud needs to be cut, there are water tight integrity issues so half of a job is in box prep - even a brand new box will leak once you start working on it and in the most inconvenient location. Without windows, they also feel like caves to me.
Here are a few more pics so you can see how this stuff comes together. I received a few PM's asking about the process and materials used so in a nutshell here it goes:
Joiner costs more at the outset, but is cut to your dimensions out of the box to fit tight in the rails provided and you will save time in installation. The wall panels are 1 inch thick and insulated
1. choose box - get the most solid box you can and prep it well. After stripping interiors go inside, shut doors and let your eyes acclimate. with a spray can walk around and mark any light you see - there are always pinholes even in a new box. Seal all holes.
2. Clean and prep the interior
3. Clean and prep the exterior
Out of sandblasting.

into paint.
4. Install insulation and foil wrap it. without the foil wrapping condensation will build up on your new clean and prepared walls. Condensation leads to mold and mildew and a funky smell no one wants to be around. As well as sub floor. This photo shows the channels for the wall and ceiling panels installed. 4 inches on the walls, ceiling and floor with 1 more in the panels
5.Wall panels going in - this is where the prep work and accurate layout of the channels makes all the difference. Putting the panels in takes an afternoon. You can see the channel at the top that holds the panels in. There is a corresponding channel at the floor as well. To install panels.
a. After interior prepped install channels top and bottom at desired location - we leave 1/2 inch gap between the insulation foil and the wall panels - this allows for air to move and keep the walls dry inside. Also leaves room for wires to be pulled.
b. The channels must be in one plane both vertically and horizontally. This is where you can account for twists in the box and straighten out your walls inside. A laser is your friend here. As well as a plumb line and patience. Measure as many times as you can
c. tack the channels in with 6 inch spacing.
d. Panels install by lifting top edge of panel into top channel lifting panel up, and sliding down into the floor channel - thats it. Kick trim will lock it in or a screw or two.
6. Finished Flooring - should be one of the last things to go in, it takes the most abuse with panels sliding around, trim tracks, welding splatter for the channels - etc. Preserve your floor and install it last. The subfloor can be seen here and is the first step in reconstruction. The openings in the middle of the walls open into other boxes with seals in between for a watertight / weather tight and insulated connection.
7. Ceiling panels go in the same way - channels and a nice tight fit. This box goes to the right of the box above.
8. Outfit as necessary - this box is a kitchen unit.
9. Pros -
a. Relatively fast - one person with all the materials and a decent set of tools (grinders, small wire feed, table saw, sawzall, etc) can get great results. I have the advantage of a blast shop and paint booth, but you can do the same in your yard with a wire wheel. without the blast and painting included there is 110 man hours on average in a box.
b. Solid - we must build these to last - I cannot afford to have a unit go out of whack while sitting in the arctic or freeze. The insulation you see here is ridiculous. you will still want some in the lower 48, but a 2000 watt heater is all that is needed to keep these boxes warm when it is -40 outside. We expect to get 15 years of service in the marine environment.
c. Multi use - all of our gear is deployable to jobsites from the arctic to the Sahara well insulated keeps warm, and cool-
Cons:
a. Cost. With the need to travel for multi use I have to build to IMO, ABS, USCG and some other specifications. That adds on 120k each unit to the build - not in materials but in approval paperwork and materials tracking from source to end point. The boxes above cost 150k to build the galley was 200k. I could do it in my backyard without agency approvals (or union labor) for 1/4 of that though.
b. The panels are unforgiving - it is extremely difficult to stretch a panel and they do not cut cleanly, so measure carefully and measure carefully again. Joiner cuts them with a water jet to the dimensions you ask for and within 10 thousands of an inch.
c. No two boxes are the same - measure every box as a separate project!!!! do not think that any 2 40 foot ISO boxes are the same, while they are close, they are not spot on.
d. Doors are where your going to leak - use good doors and forget about getting a seal on the barn doors that come on a box.
e. Transport - you will need a stout trailer - the galley above weighs 22,000 pounds dressed out. A ford F350 can do it easily, but there are only so many trailers out there that can fit a 40 foot box and fit on your buddies Dodge truck.