This is the right idea IMO but I don't want to trust a third party tool for it either (no offense OP!).
Ever since I started running coding agents with shell access I've jailed them inside of VMs. Since I run Linux I can just use incus[0] for the VM management layer.
It's extremely simple, and you can vibe code a shell script to customize your workflow in a few minutes.
No offense taken! Curious what your actual setup looks like, and whether you do any network filtering on the incus VMs?
containarium did the same thing with lxc as a container isolation and use eBPF for network safeguard. https://github.com/FootprintAI/Containarium