I wasn't trying to keep secrets, but merely:

1. Too lazy to try to explain.
2. Unsure which one will work out best.

That said OK, here are two that might work and I might be able to explain quickly:

1. A buckle that has 4 slots that are open on the end so that it can be slipped over the strap. Combine that with a Velcro strap or just use the buckle. The open ends of the slots could have bumps to keep it from slipping off.
2. A piece of aluminum (or some other material) a bit longer than the width of the strap and a bit wider than the seat post with a hole in each of the 4 corners. Slip it behind the strap and the base of the bag and then lace the 4 holes with a piece of cord tying it against the seat post.

Hopefully that is clear enough. If not I will post pictures after tinkering.