Linear expansion rates are interesting and confusing sometimes. You need to take the spec and apply it to your specific conditions, allowing also for the mating components expansion. Commonly a big deal in windows of all types, and especially in dark colors of vinyl home windows now. One of the hardest parts is that the spec for linear expansion for the adhesive is usually specific to a specific thickness of bead. In reality the spec is a sliding scale with bead thickness not a fixed value. Thick beads have more ability to stretch and thin beads tear easily. If you look at the construction of factory mounted windows in vehicles that tend to use mechanical fasteners you will find the normally would have a big hole with a bolt through it that is much smaller and the actual holding is done by big, soft, relatively thick rubber or fabric or such washers on each side of the glass. There is a discussion of one of them on a Chevy rear window now on the forum.
The problem on many of the side windows, like we have on many of our vans is that it isn't very conducive to doing that kind of mounting, so that can be harder. At least with Lexan you are less likely to have heat expansion break it compared to glass. The bouncing and such that they see going down the road makes it a tougher job for sure.
To do what you are looking at, the room you have available for bead height is probably the biggest deal, as is how to control the bounce because of the thicker bead needed for flex for expansion.
You might want to think about using a combination system of a thin or stick on gasketing for sealing, like a factory install would be and the bolts holding it to be shoulder bolts of the right gap amount with rubber or fiber gaskets on one side. To handle the bounce, a simple support on the lower edge of the glass, if it full frames so you can do it would work. On our 07 Roadtred 190 the rear awning windows are frameless and only held with adhesive in a narrow channel, but the gasket that it seal to on the window frame has a slight step on it do support the window when closed to prevent bounce pulling it out of the channel. That said, both of them had the window twisted so much to fit the van that the window didn't engage the step and didn't close right so I had to address all that.
But, in the case where others have done the Lexan replacement, they just glued them in with an appropriate adhesive and moved on. Not a bad way IMO as the risk is low. By the time the adhesive starts to fail the Lexan will probably be scratched up and discolored and need replacement anyway. Lots of them have been done this way. If you are worried about it hitting another vehicle if it fails and fall out, screw on a short tether to hold it if falls out. No need for washers, gaskets on it as a tether has no stress on it until needed at failure.
