The reason why there are not many DIY project on making your mat is because it is hard to compete with the available stuff.
You can get a solid foam mat for less than $10 and an inflatable from about $20 (E Bay...)

People like Bender make their own to have a particular size and or weight or just for the satisfaction of finishing a DIY project.
However I would guess that now Bender wished he never started that...
Same for another guy that told everyone at BPL how he could do it better and cheaper than the pros, eventually after having bought different types of fabric, foam, valves hot irons and glues he quietly gave up...
BTW, I made my own version of a Z Lite but again I could have purchased the Z Lite for less, I did it just to try out an idea not to save money.

(it is meant to supplement my Neo Air , used under it not over as in the pic)
Franco
Balloon beds are great in the backyard,I imagine you will get sick of setting them up after the third night...