Step 3: Make a platen
Measure the inside dimensions of your oven, side to side and front to back, and add two inches to each measurement. That's how big your platen board should be. (A little bigger isn't a problem, unless it makes it hard to fit your board and yourself near your oven in your kitchen.)
The easy way to make a board that size is to buy the next larger size sheet of MDF at your local home improvement store, and have them cut it to size for you. They usually make the first two cuts for free, if you buy the board, so it shouldn't cost anything and will save you a little hassle. (They don't do "precision cuts," but we don't need precision for this.)
Now make a hole 1 1/2" in diameter in the middle of the board. I used a hole saw attachment on my drill to make a neat hole, but it doesn't actually matter. If you have a drill and a portable jigsaw, you can drill a starter hole with the drill to get the jigsaw blade through the board, and cut out the 1 1/2" hole with the jigsaw.
(Don't make the common mistake of making the hole the same size as the diameter of your pipe. That creates an air flow bottleneck right around the hole, where the air must squeeze under the mold to get into the hole. You want a hole with a larger circumference than your pipe.)
Rather than cutting out a 1 1/2" hole, you could just drill a bunch of 1/4" holes in the middle of the board. (If you don't have a jigsaw or a hole saw, this is the way to go.) Start with a circle 1 1/4" in diameter, and drill 1/4" holes spaced about 1/4" or so apart, i.e., with centers about every half-inch around the circle. (Again, make pilot holes so your holes don't wander into each other.) Then drill a few more holes scattered in the middle of that. (You need 10 or 12 quarter-inch holes to avoid creating a bottleneck, with 7 or 8 in the outer circle; for a platen bigger than kitchen oven-sized, you'd need more.)
The platen shown is actually 1/2" MDF, because that's what I had around. 3/4" MDF is nicer; it's more than three times as rigid.