What I did on my marimba was to insert a length of wood on the bottom of each end panel, between the casters and the original frame, and keep the original casters.
Another option is to build a set of small, portable blocks - take length of 2x4, that sort of thing, and rest the instrument up on those. We did that a lot back in college, and we'd affix some of those rug guards on them - one on each block. I'm not sure what the exact term is - they're what you'd set the casters of a couch on, on a carpet, to protect the carpet. They help keep the casters from rolling off the blocks.
The block solution was great in college, when we couldn't do permanent alterations to the instrument, with some of the students wanting the marimbas at the original height, and others wanting them elevated. They're also good if you don't want to permanently alter a vintage instrument.
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