Here's a Rocket Express kit:
(Attachment Link)
I need to figure out how to lower it a bit. May have to go to underslung couplers as the floor is not easy to sink farther into the one-piece body.
You could mill out a cutout for a 1015 box and have a styrene shape on top to screw the coupler box to. Then either notch the end to clear the protrusion on the coupler box, or cut the protrusion back so that it clears the end from the inside and have the bottom face of the body ends serve as the "protrusion". Pretty sure that's what I ended up doing on a first-run FVM ribbed boxcar to get the ride height down.
