Yeah, I think you’re probably right about those buckets being too small for a pot that size.
Some sort of wood frame would probably work. I got the initial idea from this tutorial: https://www.cocoforcannabis.com/diy-self-draining-saucers/
I think the main thing is something sturdy enough to hold the weight. You could probably use 2×4’s and OSB to make a platform, and the maybe wrap it in vapor barrier? I’d be a little concerned about moisture using bare wood. You’d still need to find a saucer big enough for your pots though.