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Purchase a piece of plexiglass that is cut to fit inside the box that you made for your panel. You can get this from a specialty shop or from your local hardware store. Make sure you get plexiglass and not glass, as glass is prone to breaking or chipping. Cut 1 inch by 1 inch (2.5 cm x 2.5 cm) blocks of wood to fit into the corners. These should be high enough to fit above the terminal block but low enough to fit below the lip of the box. Glue these stops into place using wood glue. Fit the plexiglass onto the box so that the glass rests on top of the blocks. Using the appropriate screws and a drill, carefully screw the plexiglass into the blocks. Use a silicone sealant to seal the edges of the box. Also seal any gaps you can find so that box is as watertight as possible. Use the manufacturer’s instructions to properly apply the sealant.

Summary:
Get a piece of plexiglass. Attach block stops for the glass. Insert your plexiglass. Seal the box.