My inclination would be a plastic sheet with no features with the edges wrapped in in some sort of gasket material placed in a recess and held in place by strips on the inside held in place with screws and washers — that way if there’s a need to replace it (polycarbonate scratches easily) it’s a simple pair of cuts to get it to size.
So my goal was to reduce sound as much as possible. My fear was, that by adding clear windows I would reduce the sound proofing of the enclosure, and therefore I believed I needed something as thick as my 3/4 MDF.
At the last minute because of cost, I downgraded from 3/4 to 1/2 inch Polycarbonate…and right now, I’m glad I did, that stuff ain’t light, and thus I now realised that I needed to ask the collective for advice before putting it in the window hole.
In terms of bulletproof @CrookedWoodTex …I did knock over and smash a bottle of bourbon on my concrete floor last night as I was manoeuvring more materials for the vacuum enclosure…maybe I should be using the polycarbonate for the bar and not the shapeoko
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:
I threw this together for exactly that purpose - windows in my bi-folding doors - which provides a 5mm rebate for the perspex to sit in.
For the moment, I’ve taped the perspex into the frame, but I intend to use thin stripwood on the inside, which I will screw in place, and on the outside - if I can be bothered - I might some right-angled stripwood to cover up the (now exposed) plywood.
For all my of machines, like router tables and the like, I use Acrylic as the door. Plastic is normally quite expensive, and is essentially sold by weight. So it makes financial sense to have a frame, with thinner plastic for the window. I tend to have a glut of thicker plastic laying around, scraps, and rarely touch thin material. I machine a rebate in the exterior, and machine the door to fit inside. Couple hinges or piano hinge, and an appropriate latch(s).
The question is do you think this will ever need to be replaced. If the answer is yes then a trim nailed or screwed in. If the answer is no then sealant would be good. Either method would hold your window in. Even with sealant it would be possible to replace window but it makes it harder digging out the sealant.