Would something like this work?

https://www.usakilts.com/belt-pouch-large.html

I can't say I've had too big of a problem with sporran space when I'm wearing a tartan kilt. When I need more space is usually when I wear a utility kilt, and the clip-on side pockets are great for holding screws, tape measures, screwdrivers, wrenches, etc. That said, were I to need more space in a tartan kilt context, I would think the same general concept of a clip-on pouch would work well, and can be easily detached when not needed.