As most commercial vivs use 18mm thick panels, to compare that with the PVC boards. 2.5m x 1.2m in 19mm thickness - £384 inc vat per sheetthis time I like the look of this material https://www.cutplasticsheeting.co.uk/integral-foam-pvc/white-ex-cel-integral-foam-pvc/
at 10mm it can be structural, probably even with nutserts, but that would just be a box, if you want a basic box buy an off the shelf one, ul get a better finish for the money.
That could make a 4' x 2' x 2', with solid back and a single 60mm substrate retainer (or light shield depending which way up the thing is). If you wanted a second strip to hold the glass tracking in that would require a second sheet with a lot of leftover material.
4mm glass with edges ground all round, plastic tracking, and a couple of vents - £50 so you you could end up with a PVC 4x2x2 for around £420-£430.
If the 13mm material is used (personally I feel this is the minimum) you could shave off £123, so £300 all in
As for joining, I would probably use a solvent based poly adhesive that "welds" the two surfaces together rather than traditional screws.