It’s not too hard to set up your own VPS with Debian/OpenBSD and do the email forwarding/server yourself. The key is that it’s then literally a matter of a bash script to generate and accept one-time email addresses.

You have to jump through a few hoops for gmail to accept your emails. The documentation is there.

If you’re already running a VPS for other reasons, then this is free. If you’re not, then VPSs are extremely cheap anyway.