Step 1: Build Box
Cut the Base and Form Walls from 3/4" Melamine-coated particle board or equivalent. The melamine coating is water proof and provides a good surface to cast against. Alternatively you could use plywood that has been sealed or covered with clear packing tape, but if any water is absorbed by the form walls, the concrete will have a chalky finish in those areas.
1x - Base: 3-3/4" x 3-3/4"
4x - Walls: 2-3/4" x 4-1/2"
The size of the base and height of the form walls determine how large the clock will be. Any changes in size must have corresponding changes to the size of the Knockout in Step 3.
Place walls around base. Pre-Drill and assemble with 1-1/4" drywall screws. Use a ruler / square to keep the form walls at a 90 degree angle to the base.Step 2: Seal Box
After the box is assembled, seal all inside edges with 100% silicone. This will make the form water-tight as well as give the edges a nice round-over. Sloppy silicone work will be obvious in the finished concrete piece. Focus on making the vertical edges look good as the seams at the base will be covered by the knockout and won't be visible in the finished piece.
Tool the silicone (use something like the acrylic shaping tools
for best results, not your finger...) and remove excess with a razor blade or wait for the silicone to cure and peel up the remaining lines.
Work on the foam knockouts in the mean time.