Yo Alan - you ever order from Reliable Resin? Nice stuff, but I waited about 9 months for my parts. Others on the forum have similar and even worse experiences.
Canopy 560 glue. Airplane guys use it to glue in their canopy. I use it for gluing in headlights & taillights. I have no idea if it's strong enough to hold the trailer together.
My Camaro is done and painted in HOK burple kandy over solar gold. It's just about out of the box with the exception of adding engine detail and a set of Keystone wheels.
I use masking tape on the outside to hold the sides together. Once square, I add the top and tape that down. Then I caulk the inside of the case. I cut the tip of the tube pretty small to get a fairly narrow bead ( about 1/8").
Doesn't hurt clear styrene in any way. Make sure you clean the areas to be joined well. Some people (like me) will sometimes use masking tape on either side of the joint. All depends how good you are at caulking.