The only way around it is to dry the material in an oven before heating or gluing. This can obviously be a problem if you don't have a large oven handy. We really dislike fabricating polycarb. Usually it is limited to uses where looks are secondary to strength such as machine guards. As acrylic plastic heats, it shrinks.
Extruded acrylic shrinks one way and can expand the other.
You should test a piece if length is critical or be prepared trim the length after forming. Acrylic plastic becomes flexible at around 280 degrees. You should set your oven at 350 to 375 degrees. It will take 2 to 8 minutes to heat depending on actual temperature, thickness of material and what you place the plastic on to heat. If you need to have an exact shape for the part you want you will need to create a form or mold.