Then my pipe-based solution may be worth a look.
What I like most about it is that it took only about 2h to construct, is not in the way of my feet and the fact that it can hold other things as well, i.e.:
- large plastic/wine bottles
- small plastic/beer bottles
- a wine glass**
- a rain jacket
- a very small packet
The only things you need are:
- a 10cm (4") pipe (i.e. rain pipe, or leftover from construction zone)
- drill and circular drill (or saw)
- 1-2 hex screw(s) and washer(s) which fit the bottleholder-holes on your bike*
- sanding paper
Also, if your bike has a chain-guard, it's more likely that you don't need to put a bottom plate in there (i didn't either), as an inserted bottle (or other things) may be able to rest on a mount of the guard and be less likely to interfere with chain operation (see very last picture of instructable).
*: in case there are no holes: Make holes and get a "tap and die"-set to make the holes threaded, or alternatively drill through the frame and put screw(s) and screw-nut(s) there.
**included with wine tasting
Step 1: Idea and Pipe Selection
Someday, I needed a bottleholder for a tour and had little time to implement one. First and foremost, i wanted it to hold a large bottle, so i went around the workshop and looked for things which may fit, sawed off a piece of 10cm diameter (4") rain pipe and tried it on. But soon it became obvious that that would be a problem for smaller things, so wound up using a 20cm (8") long piece of the same diameter.
Step 2: Adjustments
Next, i wanted the pipe to sit firmly on the bike, which was achieved through two cutouts via a circular drill, as well as a hole for the (hex) allen wrench to tighten the screw.
Step 3: Final Touch
For the final touch, i removed the paint with a motorized steelbrush, sanded it a little (makes paint stick better to it) and painted it with some generic black acryl color, rated for use on plastics. Done.
As for the jumpiness of inserted goods, i don't really care, but you may put some foam or sponge material in there if you do. In any case, make sure nothing gets into the chain, which means not stuffing fabrics down to the bottom, or maybe put some sort of guard-plastic there if you see this as an issue.