I need to bond a lip around the edge of this window to sit my perspex on as when i made the tailgate i didn't add the lip. What do you think would be the best material to make a lip from? I've been thinkin about cutting some old bumpers up and bonding strips of it to the inside.

you can gets strips of rubber that are like an H shape. put that around the perspex and the other groove will hold it in place?

You could always use plexiglass/acrylic to form a parallel flat, screw,bolt it in place, then just glue your plexiglass in? Or bolts/fasteners etc.

Id do it from the inside though, then your window would sit flush.