VC,
do you know of a source. I havent found one , and was planning to make one in the future. The spacer will increase the plenum volume. It would take some testing to see what will work and what wont.
I thought of making one out of plywood or plastic. Either would be ok since only air flows through not liquid fuel. Should be an easy fabrication and 4 longer bolts and your off.
The thickness is the question, which would take some trial and error to determine, and a dyno for actual power increases. Another alternative is the seat of the pants results- how it feels. Some mods will increase the response, or "feel" of the engine with no measurable power gains.
Another project on my list of things to do- currently number 895!!!