I'm a college student on a budget, building up a Twin XL latex mattress.  For portability reasons, I'd like to keep the overall thickness less than 7 inches.
I'm 5'8/150lbs side sleeper.

How does this look for a 6" mattress? (from foambymail):
2" 20ILD latex
2" 32ILD latex
1" HD36 (39ILD)
1" Lux (55ILD)

It will rest on a thick plywood platform.

This sounds very soft.  My guess is that you will be feeling that plywood platform through the foam.

Thanks for the response.  It certainly is tough choosing the right layer thicknesses from the get-go, especially without samples on-hand.

That said, what layers should I change to get a "sleepable" 6-7 inch mattress?  I obviously haven't felt the 55ILD Lux, so I'm not sure if I'd be better suited for 1" or 2" of that layer.

Comfort
2" 20ILD
1" 32ILD

Base
2" HD36
1" Lux

I would say at least 4" of base.

I would start with at least 2-1" pieces of Lux for the base.  I'd buy 1 or 2 peices of 1" HD 36. I would spend a little extra for the R foam. I'd then buy 1" each of the latex. I know that it is thin, but you can try it out and decide what you want next to add for your full 7".  also, I'm worried that the plywood won't breath and that you'll get some mold on the bottom.

The humidity where I live is about 35% on average, so mold shouldn't be an issue.  On the website, they designate the R to mean Regular, for their 2.0lb density foam.  When you say the R foam, did you mean their 2.8lb density? 

My only reason for going so thick on the 20ILD latex top layer was because I read 1" layers were fragile.  I'm relatively light and gentle on things so perhaps it wouldn't be an issue for me?

So now it's looking like this:
one- 1" 20ILD latex
one- 1" 32ILD latex
two- 1" HD36 (39ILD/2.8lb density)
two- 1" Lux (55ILD/2.8lb density)

This should give me a configurable mattress, with 2" of latex and 4" of base foam.

You are right. I am sorry. Use the HQ foam. http://www.thefoamfactory.com/opencellfoam/opencellfoam.html.    I use the 1" for the latex. I cut the pieces in half (queen size) and they are holding up just fine with many switches. If you wanted 1" on a king, then yeah, I guess it would be fragile.  I think that what you picked out is reasonable and that it will save you some money to fine tune it. I'm not worried so much about the humidity as from the sweat from your body creating the moisture that would mold. Maybe putting some 2X4's underneath for support and then drill holes into the plywood would work fine.