Hi jkbowman,
While I can’t provide specific advice because there are too many unknowns, variables, and personal preferences involved to be able to predict which combination of materials will work best for any particular person or group of people … I can make some more generic suggestions.
I’m not sure who you were working with or purchased from previously but if you deal with a factory direct manufacturer that knows the difference between lower quality and less durable materials and higher quality more durable materials and also makes truck mattresses and can build a custom size mattress and design then talking to them in person about your specific criteria and the quality of the materials they would suggest to make sure that they are durable for the type of conditions they will be used in would probably have the best chance of success. Some online possibilities that make truck mattresses are listed here that would be well worth talking to.
If you do decide to take on the challenges of a DIY design then I would read option #3 in post #15 here first so that you are aware of some of the uncertainty and challenges involved whenever you are trying to design and build your own mattress.
Some generic comments and suggestions that may be useful …
While not everyone likes memory foam … if all or at least most of the people using the mattress like how it feels then it could make a very good choice for a top layer because of its ability to deaden motion and vibration. With an 8" mattress then 2" sounds reasonable to me.
For a middle layer I would consider a higher density polyfoam (2.0 lb density or higher) in a medium firmness level (I wouldn’t go too firm to quickly under only 2" of memory foam or it can “feel like” you are bottoming out) in about a 2" layer as well.
Finally I would use a higher density base layer (again 2.0 lbs or higher) for the final 4" of the mattress in a much higher firmness level so that you won’t bottom out on the mattress.
If you are designing your own then it may also be worth considering HR polyfoam with a density of 2.5 lbs or higher because it has a higher compression modulus which means it gets firmer faster than HD polyfoam and can help prevent bottoming out with a thinner mattress under higher weights.
It may also be a good idea to use a very firm but breathable base underneath your mattress that provides more ventilation to protect against moisture buildup in the mattress and any mold or mildew issues. Something like the bed rug here would make a good choice.
Of course these are just generic suggestions and not specific recommendations because there are many variables involved that may take some trial and error to build a design that is suitable in terms of PPP for the people that are using it. Outside of PPP though … the key will be to make sure you use good quality and durable materials that will hold up under the more rigorous conditions that they will be used in (see the guidelines here).
I would tend to avoid buying foam from Walmart since most of it would be lower quality/density that would soften or break down much too quickly and won’t perform as well as higher quality materials.
A list of the better sources for mattress components and materials I’m aware of are in post #4 here.
Phoenix