carving closed cell foam...
For the first couple of prototypes I used a very sharp hobby knife. This foam is sort of layered and you could just cut through about 5mm and peel a layer out.
For the version you see I used a butane powered soldering iron with a nylon rope cutting tip (a wedge shape) and simply melted the shape in with lots of patience.
For those that are not metricated 1" = 25.4mm, so there is about 4/5 of an inch space to play with...it does taper slightly with the front being a tad thicker than the rear area, but the foam is 20mm and fits (quite snug at the rear, seat takes a bit of a push to latch down)
You only need two tools in life: WD-40 and Duct Tape.
If it doesn't move and should use the WD-40, if it moves and shouldn't, use the Duct Tape.