Export sprite sheet with fixed frame dimensions

Is there a way to use the spritesheet exporter so that every frame is on a fixed width/height grid? I see that I can change the output texture map resolution, but what I really want is for each *frame* to be on a square transparent frame, i.e. a grid of 128x128 sprites with transparent backgrounds. I need this for a game I am working on that imports sprites at runtime for my game animation students to add custom animated sprites to.

The best way I have found to do this is a hacky workaround - I put a transparent square on the bottom layer of my movie clip, and resize my movie clip to a power of 2 (minus one, since the info panel doesn't reflect float values) so it fits on a document without adding extra padding on the right... but it's annoying!!! what if my sprite is meant to be 46x46? I was so happy to find that Animate supported sprite sheet exporting, but I wish it were just slightly more robust and customizable. As it is, I think I will have to completley rewrite my importer to support image sequences, which is.... also annoying.

Alternatively, does anyone know of a plugin that would support this functionality?