Calculate the dimensions for your isolation cabinet, based on the largest amp or speaker cabinet you expect to use. Plan to leave room around the amp or speaker, for placing a microphone and small stand. Cut MDF to size for the panels of the six sides of your isolation box.
Use glue, angle brackets and screws to assemble the bottom and sides of the box, reserving one piece for the lid. Apply silicone sealant liberally to all joints where panels meet. Allow the silicone to cure.
Line the entire interior of the isolation box with egg crate foam. Glue or staple individual pieces in place. Attach egg crate foam to the inside of the lid panel, leaving a one-inch border around all sides. Line this border and the top edge of the box with weatherstripping foam.
Attach the lid with hinges. Place the padlock latch at the front of the isolation box so that engaging the latch pulls the lid to compress the weatherstripping foam. If you choose to add casters, screw these into the bottom panel of your box. Cables from the microphone and to the speaker cabinet can be laid across the weatherstripping, which will form a seal around the cables without damaging them.