It can be made from wood scraps, Plexiglas, rubber tubing, nuts and bolts.
Sizes can vary.
You will need two pieces for each vertical side and two for the bottom of the Viewer, since the wood makes a partial frame on the outside of each piece of Plexiglas. The top of your Viewer stays open for scooping water. The whole thing is assembled like a sandwich. Drill the wooden pieces and the Plexiglas and run small bolts through those holes. This will make for a tighter fit.