No listening yet. Will do the speakon speaker-cables today (using Canare 4S11-cable).
I will use the LSR305:s as tops, until I get the horns (just some temporary textbook electrical LR24 crossover at 80Hz is my plan).
Don't know about the foam. I was recommended this mattress by speaker-people in Sweden that has measured it in closed boxes, and compared it to other damping materials.
It is this one:
https://jysk.se/sovrum/madrasser/sku...10#store_stock
It comes compressed, and kind of expands over the course of 2 days. So it sets itself a little bit tighter inside the box, I think.