Yo. I wanted my Titans Return Powermaster OP to be a bit taller, but pulling the feet straight down leaves an ugly gap. I used some lego pieces to fill them in. Works pretty well- the leg panels close, and the fillers are secure. You can easily remove then for transformation. No mods required- just friction.
It fits, but you can't close the red panel on the back of the leg when it's in. If you had shorter lego pieces with the same shape at the front, it would work.