1" of foam is good, 1.5" better 2" is great. 3-4" is overkill (cost vs return on $)
Foam is cheap as chips. Where does this extra cost come from when adding more? I don't agree with your thickness assessment, especially given that many boat boxes are against hot areas - engine room bulkheads, hull sides, tropic environments, etc. The calculation of heat transfer is pretty straightforward, and that 2" doesn't cut it for almost any practical boat installation.
Depending on box size and compressor, the electrical draw will be 4-6A. Running 24hrs/day, this is 96-144A - hardly just a "little electric" for most boats.
We have 6" of polyurethane insulation in a spillover box split into 4cf freezer and 6cf reefer powered by a single air-cooled Danfoss BD50 driving a thin aluminum evaporator plate surrounding 3/4 of the freezer compartment with a spillover fan to the reefer controlled by a thermostat. This is an off-the-shelf AB (now Waeco) system. It is in use 24/7 because we live aboard full-time. The reefer is kept at 34F and the freezer at 10F - both with a 5F hysteresis. The compressor uses 4-7A when running depending on the load and speed (it self-adjusts).
In the US/Bahamas where we are now with temperatures high 70's-low 80's, we use 40-50Ahr/day. In the equatorial tropics with temps high 90's, we use 50-60Ahr/day. I have a power monitor dedicated to it, so these are actual numbers and not guesses.
I have no doubt that our electrical efficiency is mostly due to that 6" of insulation, and no doubt that 2" would see us struggling mightily to run that system in that box in these environments. The rest of the efficiency is due to having a good flow of cool air over the condenser.
Everyone I have met with 2" of foam insulation - even with small boxes - are using so much electricity that they are struggling to keep it powered.
You will probably disagree with the following statement, and I suspect Coldeh will also, but IMO box insulation, design and placement is 85% of the electrical side of refrigeration. 10% of the remaining is removing heat from the condenser. All the other mechanical stuff like evaporator plate vs. holding plate, type of compressor, TXV vs. cap tube, etc is wandering around in the weeds.
So, a well insulated box with plenty of cool airflow over the condenser will get you 95% of the electrical efficiency possible. If those extra lost 5Ahr/day are killing you, then throw up a small solar panel before investing in expensive stuff like water cooling.