Seitz window in tailgate

[v-lux]

So what do people think would be the best thing to mount one in?

1 - I could spend some time (or money) trying to find a solid tailgate. (expensive and/or could be waiting a very long time and end up with a very thin plate to mount the window into)
2 - Get a panel welded in my tail gate. (Also end up with a very thin plate to mount the window into, will need welding, painting etc. Not easily reversible)
3- Find a tough, stiff sheet of something that will fit in the window rubber. (Reversible, a bit thicker than a steel plate would be, maybe not quite as secure depending on the material?)

I prefer the idea of mounting it in like the one in the photo.

What material could do this? I've looked at polycarbonate sheeting, like they use for vandal proof windows etc. Pretty sure it would be tough enough, but i'm not sure how flexible it would be?

[Plasticman]

nice bit of alloy plate cut to fit,will be as secure as a window was. 'U' gasket on the alloy to build the thiuckness up if required, same as i do for louvre windows in van coinversions.
also stiff enough to cut a window into as well
mm