I made the side panels by laminating five sheets of 5mm MDF, PVA glued together & screwed to the front and back panels, the internal midrange and tweeter enclosures (thus sealing them) and the internal bracing. I was careful to make sure that I got as airtight a seal as possible, by caulking with a hot-glue gun after the PVA had dried. This did take a bit of time and care.