Warren, If you havent glued the roof on, Ill send the supports out to you. I have a bunch of them here.
Use the solvent to glue the styrene on, it works wonders.
Id sand down the extra length now. One of mine was long like yours, and I used a box knife to trim it down. The trimming doesnt have to be real neat, as the corner parts will cover it.
Bruce “Acrylic” Chandler told me a neat trick for the solvent. With the bottle upright, squeeze the bottle a bit, THEN turn it over, so the suction is keeping the solvent in… You can then squeeze/unsqueeze the bottle and control it much better.