Well, I have a couple of tank bag none of which is a clip on. But they use straps to attach the support to the bike. One of them is a magnetic one (of which the magnets were removed) and is permanently attached to the tank.
On this support the real tank bag is attached.
The support can be used as a card holder. As this has been bought when Airheads were everywhere, it has an opening for the tank filling....
If yours has none, you can cut one and sew the edges not to mar the paint. 8-)
Bear in mind that the BMW Multi Vario was based on this design and very expensive, then.