Nan, I LOVE your idea, but with a 75# dog, his "deposits" are pretty big and I'd rather keep them as far away from my body as possible.
It's bad enough having to pick up so much, you know!
All of the mainline pet stores carry those bags, usually in packages of 6-8 rolls. I've been using them for 4-5 years now (maybe one of the benefits of living in a metro area). Instead of attaching the holder to Jed's leash, I just take 3-4 bags from the roll & loosely tie them to the handle. By the time he's done his business I've untied a bag and am ready to pick it up. Works like a charm for me, and I don't have to worry about transferring the container from one leash to another. I use a retractable around town, but a 6' leather leash when camping. I can tie the bags to the hard handle and to the loop on the leather leash. Quite colorful!
Actually, I've got another little pouch that attaches via velcro to his leash that I can keep bags in. One velcro loop goes through the handle & the other attaches a little lower. By putting one strap through the handle it doesn't slide down the leash (does this make sense?). I don't use it very often, but when I find it I'll take a picture of it so you can see how it works. Nan, I'd love to have something smallish that could be attached to the leash to keep filled-up bags. That way I wouldn't really have to carry it. But it'd have to be fairly good-sized -- he's a big boy!
Laura