have you ever just gone to a store to purchase 3/8" thick Plexiglas? it is expensive.
that's what the rulers are made of.
and, they have to be thicker than your cutting rulers so the 'hopping foot' does not go up over the top of them- breaking needles, causing all kinds of bad, bad things.
a good source for them is the dealer who carries your long arm machine- they often have specials and offer deals because you have one of their machines.