does your negative view holder take 5 across or 6 ?
just trim the bottom ( or top ) 5 or 6 frames ( or 10 ... 12 frames ) off and hang them next to the roll.
put the weighted holder or clothes pin on the sprocket holes to weigh it down
so it doesn't curl.

the film can be trimmed on the reel ...
no mess no problems.
unless of course the cats are in the tank ...