I have a moonboard sized wall and use a matress and two crash pads.
I can definitely bottom out the matress if I'm not careful, a sheet of minicell over it would be ideal.
The two pads I already had for local highballs. One is a high quality pad, the other is a beater I bought used.
it works for me and gives me good flexibility, just means packing it up when I go bouldering.
My wall is about 15 feet high, i.e. the same as a commercial bouldering wall which is high enough to hurt yourself so I just coughed up and bought a proper crash mat from here https://www.foams4sports.co.uk/product/promat-crash-mats/
which cost me about £250.
My reasoning (as well as not wanting to break my ankle again) was that two of us use the wall and payed £125 each which, including fuel, pays for itself after 13 uses when I would have otherwise driven to a wall.
Annoyingly, about a month after I bought it, the PE dept at my wife's school had several surplus crash mats to get rid of so I could have saved the cash. Still, I've got 2 crash mats now!