acquired some offcuts from awesome walls ...awaiting cutting in the shed! Good recycling tho 👍
This worked pretty well, cutting the sides was easy enough with a panel saw. And once it's inside the covering all is forgiven. Getting the depth cut was more tricky but still doable (I was going from a piece of bouldering wall flooring (say 25cm) to 11cm. I added a bit of extra depth over the original and nibbled a bit off the sides to accommodate the extra volume.
The foam was in pretty good nick really. Not an option generally I guess, just got lucky on the timing of AW replacing/remodelling. The DIY wasn't hard though.