If they have very sturdy rests already,
you could strap milk crates to one to get
it nearly to the right height and then
use something softer for the final bit.

Apart from needing something firm to rest your
FOREARM on (ie NOT the rifle), the other important
part is having something solid to push your thigh
against (or lower part of your body in general).

This also tends to aide stability for shooting
while not stopping free recoil.
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