I guess I am cheap. I use the inserts from the Atlas-O track boxes. Both the 40-inch straight and the curved section boxes have nice inserts that can accommodate almost any engine with just minor shaving of the edges. I have worked on several steam engines, tenders, and large diesels and turbines. Here are a couple of examples:
No. 80 Steam Turbine
Vision Challenger tender. The block on the side of the tender (plain side up) was just right to rest the tender frame when disassembled (but I did not take a picture!)
Alex