Not a bad idea but you would be much better served by an Engel - it uses half the power of your choice - or less. More expensive but the most energy efficient fridge of its type available.....
I wouldn't remove the shell but would insulate around the unit - or purchase one of the insulated covers that is optional.
The Engel is a great portable fridge and it is true that it runs on fewer amps than a danfoss type compressor but that doesn't necessarily make it more energy efficient.
It uses a different type compressor/motor, what is called a 'swing motor'. It uses fewer amps than the danfoss, but from all that I've read runs longer to cool the box. Most sites say usually more than twice as long to cool an equivalent size box to the same temps. In reality they are very close to each other in energy used.
Here are two links to some comparisons and the second has the most info, but takes the longest to read through.....
Ray's Caravans, Campervans & Motorhomes - Portable Fridges 1-
On the first they list daily amp/hr usage for the Engle at 50 and 42 for the Waeco but I think the Engle is a slightly larger box to be fair. Anyway pretty close to each other.
We have a 63 quart EdgeStar that has a Danfoss copy compressor/condeser and is like what the OP is looking at. It lists at about 5 amps, but that is start-up and we usually see 3 1/2 after a few minutes.
We have used it about 3 months out in Florida. One in the cool fall/winter and 2 later in the late spring when we were getting 80 deg cabin temps. It was using in the 30-40 amp/hr range on the trip and we would add a few hot drinks every day. I'd say it is right in there with the ones in the report.
Saying that if I was to put a fridge in the MacGregor again I'd make one like we did for the Endeavour....
Endeavour 37 Interion Mods Index
....It is larger, has a small freezer section and uses less than 1/2 the amp/hrs that the portable does due to the increased insulation. It would be hard to insulate the portable since you have the openings on the end for the compressor cooling. If I remember right they also want the exterior walls to not be up against anything.
The insulated top cover would help and we put towels on ours, but the bottom is actually where more insulation will pay off as that is where you have the biggest temperature differential between the inside air and outside air in most cases.
My suggestion to him has to make a well insulated box and...
..... buy a good compressor/condenser/evaporator if he wants low energy consumption,
Our 37 Endeavour --- Our 26 MacGregor --- Trips With Both