I made mine at http://techshop.ws
sheet of 1/2" plywood
Step 1: Determine the Maximum Dimensions of Your Box
In this walkthrough I'm making a box with sides 2' long and 4" inches tall. That would potentially let you cast a 2'x2' square, which is plenty for small projects. If you're trying to bullseye a specific size, keep in mind that the internal size of your box will be slightly smaller than the length of your walls because you will lose at least 2" where each wall overlaps its neighbor in order to have a place to clamp them together.
Step 2: Measure Out Your Cut Lines
Step 3: Attach 2x4 to Plywood
In each of the sections indicating a wall height, drill two pilot holes. Be sure that each hole is heading through the plywood and into the 2x4. It is usually best to offset them from each other to help miticate the 2x4's tendency to split. Countersink the holes you drilled so the screws won't stick up above the surface of the plywood. Screw screws into each hole, making sure the screws are either flush with or below the surface of the plywood. The plywood should now be VERY firmly fastened to the 2x4.
If you want a smooth surface on the outside of your silicone mold, fill in the tops of the screws with wood putty or an epoxy. Left unfilled, these will leave little protruding 'X' shapes. They don't really bother anything, and can be easily cut off with a sharp blade after each mold has been made.
Unclamp everything and carry your wood to the table saw.
Step 4: Make Your Cuts
An optional step that will make things line up really nicely is to move your fence 1/16th of an inch towards the saw, spin the wood 180 degrees so the 2x4 is now on the cutting side and run the piece through again. This ensures that the plywood and 2x4 share exactly the same edge, and that any warped edge on the 2x4 has been shaved off.
Set the saw fence to cut at the desired height of your walls (4") and spin the wood 90 degrees so that the end of the 2x4 butts up against the fence. If your measurements are correct, your cut will pass between the screws you used to attach the plywood to the 2x4. You should verify that things line up correctly before you make the cut. No sawing through screws. Don't. Really.
Make a complete cut. You should be left with one now-complete wall (with a chunk of 2x4 on one end attached by two screws), and three still-attached wall pieces.
Repeat this process three more times, once for each of the remaining three walls. Check each time that the path of your cut through the wood does not pass through any screws.
Step 5: Clamp Your Walls Together Into a Box
Place a second wall next to the first wall in the same orientaton, and on the 'inside' side of the first. Rotate the second wall 90 degrees using its 2x4 as the pivot point. Slide the second wall down the length of the first wall until they form a 'T' shape. Clamp the second wall to the first wall using the second wall's 2x4.
Place a third wall next to the second wall in the same orientation as the second wall and to its 'inside' side. Rotate it 90 degrees as before and slide it down the length of the second wall until they form a 'T'. Clamp the third wall to the second wall. You should see a pattern emerging.
Add the fourth wall and clamp it to both the third wall and the first wall. You now have a rectangular box whose dimensions can be adjusted by loosening the clamps and sliding the walls around in relation to each other.
When casting, place the box on a piece of cardboard to protect the surface of your table and to ease the removal of the mold.