0Re-design 8 years ago ReplyUpvoteUse 1" insulation styrophoam from the home builder supply. Cut it to shape and pin it together with 3" framing nails. Then tape it together at all of the joints with aluminum hvac tape. It's strong and sticks great. Duct tape won't work unless it's the best quailty. Make it double layer if you need more strength/ insulation. You can build a very tight box this way.