Sure' date=' they're not too hard and look really cool.
You'll need a drill press, 1.75" hole saw, drill bits, a 6.5" round Microplane rasp, a flat microplane rasp, a jigsaw or bandsaw, a 3" #10 machine screw, nut, and wingnut, and a small scrap of 14" inch plywood.
Make the sheaves out of 1/2" UHMW polyethylene or UV-resistant VHMW polyethylene. Cut them out with the hole saw, which will give sheaves just over 1.5" in diameter. Then chuck them into the drill press using the machine screw and a couple of pieces of plywood cut out with the hole saw. Use this like a lathe, and turn a groove into the sheave with the round rasp:
[img']http://216.77.188.54/coDataImages/p/Groups/147/147689/folders/113785/795896sheaves2.jpg[/img]
Use 1/4" brass rod for the pin. You can then drill the sheaves out to 1/4", or out to 5/16" if you want to use an oilite bushing. Brass rod, 1/2" polyethylene sheets, and oilite bushings are all available from McMaster-Carr, http://www.mcmaster.com
For the bodies, the easiest way is to use a piece of douglas fir 2 x 2. You'll want a block about 3 or 3.5 inches long. Draw a curve on the top and bottom. Mark where the pin will go through; you'll want it to be offset about 3/8" off-center toward the bottom to allow the line to pass over the top. Drill a 1/4" inch hole witht he drill press.
Mark the opening for the sheave on the face perpindicular to the pin. Mortise it out by drilling a 1/2" inch hole at each end, cutting the waste with the jigsaw, and shaping and enlarging the mortise with the flat rasp.
Now use the rasps to shape the body of the block and cut a rope groove around the outside. Sand smooth.
Finish with either an oil finish or varnish, splice an eye to fit around the block, and tighten it up with a seizing of tarred marline:
Tie the tail off to the boom where needed: