I recall someone else came up with a rather elegant solution: swap the underframe for one taken from any of the FMC boxcar variants made by MDC/Roundhouse (and now Athearn)
I've been meaning to try this for myself, so I'll muck around on the weekend and post some comparison shots.
OK, so I've now tried the MDC swap and decided it wasn't worth repeating, let alone taking a picture: after filing the MDC underframe to fit and shaving the inside of the TLT body I ended up with a car at almost the same height as a stock TLT model.
I think the Atlas FGE boxcar frame also fits these and also FVM boxcars... I have to try the MDC solution!!
I also looked at the Atlas FGE underframe, however it is significantly wider than the TLT frame and would need a fair bit of trimming to fit.
Now to get some BLMA trucks and body mount couplers so I can try Bryan's idea...