Hi testarific and welcome to the Mattress Underground 
Looking to make my own mattress for the back of my minivan build. I’ve been reading through this trying to figure out what elements to purchase and have been overwhelmed by the options.
Details: Height: 6’, Weight: 140 pounds, Male, 100% side sleeper (have tried transitioning to back but could never fall asleep)
Budget: no limitations, would like to spend <$400
Bed size: full, will need to be <=6" (I’ll be cutting it down to 46x72x6 in the end)
Very cool project! With what you’ve stated, with your lower BMI (thanks for providing your Stats!), budget and dimensions, you should be able to check all those boxes without too much trouble.
Anyone have any suggestions for layers? I planned on a basic 3" of HD36-HQ and then 2" of ViscoPlush Memory Foam. If one were to add in a 1" layer of 29ILD latex, would you just toss it on the top, or would you change out the memory foam layer for something else. I don’t necessarily need the latex, but am open to it. I could also go something like 3", 1", 2" layers if there’s an option there, but really just looking for advice for for a thinner DIY bed.
Here’s where the personal preferences part of ‘PPP’ come into play…with your BMI, it comes down to how you like the feel of different foams and their respective firmnessses. Generally the choice between latex and memory foam depends on your preferences and this is an area where no one can tell you which one will be ‘better’… The HD36 HQ is a good supportive base layer; the memory foam - if you like the feel - and if you are keeping the thickness down to 6", + 3" of visco plush memory foam could work but there are some draw backs. For a mattress this thin, with a 1" latex topper and a 2" visco plush, you really wouldn’t get much effect from the latex. The trouble with RVs is that the wide range of temperatures create condensation and memory foam is notorious for developing mold. Latex on the other hand is mold resistant and antimicrobial which would be a better suited material for an RV. Additionally memory foam changes firmness with temperature; In cold climates it starts feeling firm and as the night progresses the heat is trapped and softens the foam. So what you start with at the beginning of the night might be different than in the morning.
Do keep in mind, as you can see by comparing the Mattress Specifications You Need to Know to the Mattress Durability Guidelines latex is one of the most durable materials you can have in a mattress. If you enjoy the feel of latex and find the right comfort level, a 3" latex layer above the HD foam base would probably be the best. If you are leaning toward memory foam, we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support. Memory foam, if made of insufficient density can cause the ‘divot’ effect you have with your Ikea mattress, and using Low density foam as a transition layer for your mattress may cause issues down the line, such as aches pains or even alignment issues. A latex topper would also be a possibility, but again, a 1" layer would not really add any significant change in your comfort level.
I will say that several of our Trusted Members, such as Arizona Premium Mattress and Oklahoma Mattress Company specialize in creating custom RV mattresses for consumers; by giving them your Stats, PPPand the feel you want to achieve they can take the guesswork (and footwork!) out of your RV sleep solution, and would allow you to focus on finding the right support/comfort fit. We look forward to hearing how the project turns out!
~ Basilio