I have exactly one and a half (1.5) reloadable Mamiya 16 cassettes and might be able to provide dimensions. Not all of their film cassettes were designed to be reloadable, so beware if you manage to source unopened cassettes with the intention of easily re-loading them.
The cassettes that are reloadable have 'handed' caps for the take-up chambers. If you put them on backwards (yes, they fit) it seriously impedes and scratches the hell out the film.
The barest way to put film through it would be to use 3/4" length of dowel, about 1/2" diameter as a take-up spool with a notched end to engage the advance mechanism. Add a flat cap on the other end to maintain its vertical orientation within the take-up side.
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