Or just an idea if you can't find them / can't wait
spread some Neo-Rez wetsuit repair goo on the bottom of the footstraps where they will contact the board. Let it dry well before screwing the straps back on though!
it sets rubbery and sticky - really resists the twisting once it is pressed down hard onto the pads
I find mine don't twist, just using the rectangular washer with teeth under the screw head (not under the strap, that may be bad for board cosmetics
haha)
but the Starboard thingy described above is very good - wonder why other makes don't use it also
