so the motorized bike 2L tank fits in the rack. It has 4 studs on the bottom of it so that you can attached two brackets around a bike frame. Of course, we don't have that on the Express unless you were to try to mount it on the frame in the front which could be a possibility. The studs are about 1 1/2" long and make it a little difficult to get inside the rack. You could cut them down some if you wanted to make it easier. Anyhow, I'll work on a solution for securing it to the rack and maybe look into mounting it on the frame and report back with pictures.
Btw, get one with the petcock included. I didn't so now I have to buy the petcock separately and make sure it's the right size.