Current Mattress: Tempurpedic Luxe Adapt Soft
My specs: Female, Spinal issues/pain, 90lbs, side and back sleeper bony joints, need cushioning and pressure relief. I do spend a lot of time lounging on the bed on my back.
Problem: Sink too deep and feel the “hard” layer of the base underneath. Creating DIY “Bunkie board” made of foam to relieve this, which sits on top of plywood. Do NOT want to use a topper and add to bed height. Need it to sit 1.25in height AFTER the mattress sits top of the foam, so taking into account the compression of the foam will determine the foam height I need…
Tried: 1 inch 50 ILD, mattress does feel softer than the plywood, but not enough. Still feel a hard layer underneath.
Trying to figure out what ILD/density, foam type (HR vs poly, low vs high density, or even latex?) and height based on the foam I choose would work best. Don’t want to go TOOO soft so that the mattress will just compress down the layer I add.
Does anyone know the ILD/density specs of the polyfoam base layer on this mattress? I believe if the ILD of the foam is less than the base layer already on the mattress, this will be counterproductive?
Considering a 2.0lb/28ILD or 35ILD Lux foam, 1.9lb/33ILD foam vs 2.5lb/28,34 or 40ILD High resilience foam.
Would the Lux vs High Resilience foam be better for helping the mattress not sink as deep (perhaps spring back?) and giving it a more balanced feel?
I’ve heard about flipping the mattress over and using a topper, I don’t want to do that. 
I’d appreciate your thoughts on which of the foam ILD/densities, and foam type above, could be best recommended. Thank you.