First of all, don't use plexiglass. It fractures easily, and not permitted on the robot. We use MEK (methyl ethyl ketone) as a solvent glue for Lexan (ploycarbonate plastic). If the part is under stress, we sometimes use machine screws tapped into the joints for extra strength, along with glueing. For a good glue joint, ensure the surfaces are clean and flat, and clamp until the solvent has evaporated, and the joint has solidified. Hope that helps.
