JIC.. I spotted this after running into same problem -- (Robert from Blue Planet Surf being generous with his knowledge)
So.. If you DIY this may help.. (if nothing else it is a nice watch)
I had trouble sourcing a block of EVA locally - so ended up sticking 4 layers together of softer compound which I could hack-saw but not easily sand.. the trial chop job plus 2 screw holes took 20 minutes

(not fine art - but first effort went well - kite foil board - a bit lighter to yours)
I have just been alerted to Knauf insulation foam Board which will probably do the job that EVA in the video does..
For Bunnings $20-25 you will be able to get a generous sheet 50 mm thick which will be good for at least 20 prototypes..
I am about to build a quick proof of concept board (kite-foil) and will have off-cuts to use for foot hooks - so will report back if the strength is there to just cut drill and use on the board.with no further fuss.
Cheers
AP