In a nut shell. here are the results of an article done by Summit Racing on the "thermal conductivity" rating (TC from here on) of various types of carb spacers. (See dictionary or Wikipedia for definition &/or more info.)
1. Wood- Best absorption/insulation in their tests, the TC was 1.02, down side is possible damage to wood by fuel over time. 2. Phenolic Resin- 2nd. best heat resistance with 2.01 TC rating, more resistant to fuel. 3. Polymer- 3rd place with 3.90 TC. Although some polymers have been developed that come close to phenolic resin. 4. Aluminum- Last place with a TC of 1,665 (not a typo). Ouch! That's hot!! I couldn't find any info on the nitrile rubber base plate gaskets so if you have info. on these please post. Also, if you've used wood/wood composite spacers how do they hold up?