Step 1: Materials
Camera Body Cap (Available at ebay for Nikon and Canon).
PVC pipe slightly smaller in diameter than the body cap (I used 5cm / 2" for nikon cameras).
Sheet of foam (Ethylene-vinyl acetate) 5mm / 1/10" thick.
Mate black spray paint.
Crazy glue or any other cyanoacrylate based glue.
A ruler, a drill, circular saws, scissors, hot wire and other cutting/measuring instruments.
Step 2: Cut PVC pipe and spray paint.
Keep in mind that the longer the PVC pipe the more magnification you get from the microscope. Pipes longer than 10cm / 4" will require exposure times grater than 4 seconds (under ISO 200).
After you've cut the pipe make sure the base and the top are straight by sanding the edges. Once you have a pipe that will stand straight on a flat surface you can start painting it.
Remember to use mate paint, otherwise the glossy finish will cause reflections that will show in your photos. While painting apply several thin layers of spray paint.
Step 3: Cut and glue foam.
Once you've got your ring, apply a small amount of crazy glue to one edge of the PVC pipe and glue it to the foam while applying pressure.