I've seen a fair amount of interest in plenum spacers lately but at this point they are pretty hard to come by. kmperformance only does them in batches and its hard to organize even a few people to get them in a timely manner. I didn't want to wait so I bought a $20 piece of Delrin off ebay. Yes that piece cost $20 shipped. 1/2" thick.
I just layed it out by tracing the gasket. A very fine sharpie works great. (The piece in the picture is 2 5/8" x 24")
Then I drilled the holes out with a 1 1/4" hole saw. The actual hole is 1 1/2" but I didn't want to make them too big. I just made them the right size by "porting" them with a die grinder with a carbide cutter. Here you can see How I started to get the rough shape with a 90 degree cut off wheel.
After getting the rough shape down, I just went around it with the die grinder and smoothed it out. If you really want to make it nice, a dremel with sanding bands would do the trick.
I'm very happy with the results for the price. It does take some time if you want them to look nice. I've got about 3 hours in each start to finish but they could use some dremel work to make them look really nice.
If you decide to do this I would recommend Delrin or something that can withstand engine bay temps. There was a picture floating around on the Stealth/3000gt forums of a spacer made out of ABS plastic and it was completely melted! Delrin is a proven winner, Certain types of Nylon have been used as well as Wood. Steel and Aluminum could be used but you don't gain the insulating properites of the other materials which help keep the upper plenum cooler. Also it has been said many times that P-bodies can only take a 1/2" spacer due to hood clearance. Even with 1/2" you may have to remove some the the underhood "skeleton". Keep that in mind!