Wow, that bike bag is HUGE.
First, I got all balled up at the corners because I was fitting half moon pieces into straight pieces. Not my usual "so tidy on the inside" at all. When I turned the bag right-side-out however, it looked fine so I guess I can live with it.
Next, I saw that the straps or loops were too small to put buttons on so I went for some left-over snaps I had. I think I may go for snaps in the finished product also. Then I stuffed the bag FULL of every hand towel I own (it is very capacious) and went to the garage.
As I planned, I slid one fixed loop over one end of the handlebars and then slid it to the middle to get the other loop over the end of the bar. That went well.
Then I centered it and snapped the middle loops shut and there it was - the biggest handlebar bag I've ever seen. I think it will connect with my knees as I pedal but I won't know for sure until we go for a ride or at least until Peter holds the bike for me while I sit on it. It sure didn't seem very big with the dimensions I wrote down, nor while I was making it. So I am glad I made this template and will go back to the drawing board after a test drive.