This called for a proper porteur bag, but it took me a long while to get there. I'd use bungies and all sorts of bags, none of which looked particularly nice. Having my bikes look nice is kind of a thing for me.
(this looks tacky)
Then Lesli gave me some big pieces of waxed canvas. I could make a bag. It spent a couple of years being designed in my head, and by then I'd acquired some Pendleton blanket-weight fabric (do NOT ask how much it costs per yard, even at the outlet store. Just don't.), and my sister-in-law, who runs a halfway house for textiles*, came into some fluorescent green and pink Cordura, which she figured I could find a use for.
I've been collecting bike helmet buckles and straps (WELL WASHED) from helmet giveaways (we confiscated the old one so it would not be used again). I certainly had the pieces.
I wanted a fully-lined bag, with some internal pockets for the bits that get lost in the big compartment, and a pocket on the outside for my u-lock and cable, because it takes forever to attach and remove it from my rear rack. The lining had to be removable (for washing), and the bottom was to be stiffened with a piece of Coroplast (I have a small stash of old lawn signs just for this purpose)
The strap - well, I could BUY webbing, but what with the waxed canvas and Pendleton fabric, it called for an exceptional strap. I tossed my yarn stash, and wove a strap to match the Pendleton fabric, because I could.
Measured the rack. Measured the distance between the handlebars - that was a constraining factor.
Made a paper pattern and binder-clipped it together (best sewing accessory since the metal needle, although the Clover Wonder Clips may step ahead in line).
Stared at it awhile. Looked at other bags on flickr, in particular, David Parsons' bags.
The original design:
Basically, the same size as a reusable grocery bag, but an inch deeper.
Sewing it was fun - my Bernina 930 was certainly up to the task, but arguing with the heavy fabrics did prove a bit of a challenge.
The lock pocket, which was a simple flat pocket, decided it wanted to be a bellows pocket, due to the depth of the lock and cable.
The bag clips to the rack with helmet webbing adjusting clips (Bern and Bell), and running the strp through the tombstone before clipping it; works very well.
And, in use:
Notes for next time (hah)
Make it taller for better roll-topping, and use a wider strap.
On the whole, though, I am pretty happy with it.
*textile halfway house - fabrics in interesting quantities show up on her porch. She redistributes them. Anything from a bolt of raw silk upholstery fabric (pajamas for me!) to the Cordura, to ribbing, to...