Peltier devices are designed to fail - out of safety the matrix of active junctions are made to eventually open up instead of welding into a short. IF you are talking surplus elements one can't tell how they were sorted or why they are surplussed. 600-800 operating hours is a very long lifetime for a peltier chip - but only a couple of months of sunshine.
Opps, you blinked... We just entered the trade secret realm - proprietary knowledge zone ... I earnestly forward to you reverse-engineering 100,000+ man-hours of trial and error done by Universities and Government Research Centers is not going to save any money for a aspiring basement DIY panel maker.
Can't say I know any of the following more than intuitive extrapolation from a lot of reading...
Roughly a panel is Glass - EVA - Cells - EVA - Tedlar.
There is a 'tackifier' coating to the EVA laminate sheet that touches the glass in mass production that promote adhesion to the glass - on the other side it has another coating that helps weld EVA-EVA together when heated and subjected to vacuum clamping.
EVA is close to the original hot-glue sticks composition, sticks being a cruder resin. base. Dupont has all the materials you need - but they want to sell you 5-ton rolls.
"Etching" from water can occur without visible moisture, a hermetic seal is vital - then there are mosses, lichen and algae and things like soot and pollen that would rapidly take the performance down.
Glass is to be iron-free, iron gives glass edges a green tint & absorbs energetic wavelengths.
Your PVC sheet most likely has too large of a coefficient of expansion to be used - freezing to 140°F should be expected and the heave and shrink of polycarbonate is too active to be adhering to fragile PV cells or trapped tightly in an alloy frame; plus the vibration resonance on impacted by hail or debris might expand any flaws present or help lift tabs etc...
An oven can be a recirculating loop of forced air that a heat-gun adds heat into to keep the heat evenly distributed over a cycle period, the return air duct monitored for temperature along with a non-contact IR thermometer spot checking surface temps area - drywall or cement tile backer board with plenty of fiberglass insulation comes to mind as oven shell material.
I've seen silicon wafer strings layed out face down directly on tempered glass in aluminum frames that were immersed in a vat of nitrogen gas and silicone sealant poured in as encapsulation then heated to set the silicone - that is old style marine navigation arrays.
-------- oops, chores calling... I will continue later ------