Hi,
I bought a tank bag from off-the-road, and am pleased with it - although I 've not used it on the bike loaded up.

It does impinge slightly on the rider, but with a realively small tank area, I think most tank bags would.

Fitting wise, you attach 2 straps (1 either side) to the frame, just below the seat (these can remain on the bike at all times, just tuck the tag under the tank).
The bag base then attaches to these to straps, and one strap around the headstock. The bag then zips on easily.

I can get more specific photos if you are interested.

Russ