Why not just use a bucket? they've never breached my ridgemonkey yet. I never leave my bait in the bivvy either. And always close my food bag, then I throw boilies in other people's bivvies works a treat 😂
Had two 2.5l lockable sandwich boxes delivered yesterday which should keep them at bay. I fish several different waters over the course of the season and I'm sure they're getting bolder on all of them!