I have a very nice little Evoc frame wedge that sits happily under the top tube, secured to the seat tube. The curvature of my frame is such that I won't get one in against the head tube.
Or rather, it was nice until both zips decided that they will no longer zip up, and have burst open. Never to shut again. Evoc has kindly sent me a new one under warranty but given it is the same style zips on the new one - I'll be looking for a new one in a couple of months, I'll wager.
I'm aware of a little Zefal one... but am looking for 1.5L to 2L - and only want a bag that fits in that position on the frame - Not Piggy's, Frame storage bottles etc.
They seem very hard to find. Anyone?!
Alpkit Wombat? No idea if it's good or not, just that it exists. If Tailfin ever make one, get that. Their top-tube bags are excellent.
https://alpkit.com/products/wombat-small-frame-bag?variant=39979916951657