I've got a couple cats with damage, one of which has two areas ~1' sq that I had to cut completely out (1 sq ft after grinding, cleaning up, tapering, etc).

I'm curious what my options are if I want to try to include some core material in the repairs. I feel like I'm going to add a non-insignificant amount of weight if I make every repair solid, so on these large two I'd like to save weight if possible. However, I don't have any rigid foam, and these areas are fairly curved, so it seems like a rigid foam core would be somewhat labor intensive.

Are there liquid alternatives? I'm thinking a glass bead fairing filler in epoxy would have all the properties I need, aside from being expensive, but I have tons of it. I also have expanding urethane foam, but I haven't seen any info about using it as a core material.

It also looks to me like some balsa core sheets are flexible (like 1"x1" tiles)...if that's the best way to go I can do it, I'm just trying to avoid ordering stuff.

I know these are likely fairly basic composite questions, but I've never done any layups with cores, (just simple repairs and cradles) so I don't know what's possible and what isn't..and I don't have any core material on hand.

Thanks for any advice!