Hi, I was hoping for some advice on materials for a DIY mattress for my parents. Thanks in advance for any suggestions!

Requirements:

- VERY firm. They like beds firmer than most retail stores offer. They have slept on thin cotton pad over a Tatami mat before and found it perfectly comfortable.

- Reasonably lightweight base as they do not like having heavy furniture. 

- They are small people (both weigh less than 110 lbs) and they are thin. 

- This will be a full size bed.

I was thinking a Lux-HQ base with a soft or medium latex topper.

 

How thin of a base can we get away with if this will be on wooden slats from a futon mattress frame (we MAY reuse an old box spring on top of the slats if that works better). Would 4" be OK for a base or would we need 5 or 6"? 

For the latex topper, would a 2" be sufficient, and would 20 ILD or 32 ILD be better on top of the Lux-HQ if they want a very firm bed? 

Any other suggestions for materials?

I was planning on stuffing a futon mattress cover with the foam and putting the latex on top, and then a mattress protector over the whole system. Would that work OK?

Thanks very much!