Question about spacer between brake booster & firewall

Okay, swapping in a '96 system into my '91. I have everything w/ the exception of this spacer. I was told it's a 1/4" or something like that and w/out it, the booster rod won't line up to the hole on the brake pedal. Went to Jeep and system only showed spacers for 92/93 and it didn't matter b/c it'd be the wrong booster AND it was a discontinued part. The guy said that it was basically a gasket for that year, but didn't know about a '96. So, my question is, was it metal or a spongy type material? Was it even a 1/4"? Since I'm at a dead end here, I was thinking of making one out of a mousepad if it indeed was "spongy" and only about 1/4" thick. Sound like it'd work? If not, would a 1/4" piece of wood work? I'm sure as long as it's the right thickness, I'm fine, but didn't know what the purpose of the spacer was - for a seal, eliminate vibration, etc.