For soundproofing, it’s not thickness that matters but mass. Polycarbonate is significantly more dense than MDF so you can get away with it being a bit thinner. For my Nomad, I have 10mm thick MDF walls plus 6mm thick polycarbonate windows.
That’s some serious plastic. I also have 12mm of polycarbonate but for soundproofing I’ve got two 6mm sheets with an airgap between them.
As for the original question, I second @WillAdams’ suggestion. For Aluminium profiles, there’s this stuff:
It slides into the slot and holds your glass/polycarbonate for you. I think it’s a soft rubber so shouldn’t transmit too much of the sound.