My #20 was kitbashed from the Carter Bros. "Master Class" we did here, which is a 13-window board-and-batten coach very similar to #3, except for the roof ends (Carter Bros. used a duckbill end, while #3 had a more traditional bullnose end) and #3 has tongue-in-grove siding below the belt rail instead of board-and-batten.
I've got the drawings for that car, and can send them to you. You should be able to adapt that for your use.
Question--can you cut 1/8" steel on your laser cutter? I'm working on a model of EBT #5, their 2-6-0. I've got a guy printing my wheels and another guy turning the tires. The guy who was lined up to cut my frames has dropped out of the project.