There are other options. Velcro ... if you want to take the chance it wont come away.
Strong magnets ... if you don't mind how they might affect electronic equipment, credit/bank/ identity cards and so on.
I got a cat carrier-basket and mounted the base on to the back of the bike. I could just as easily have used an old tool box, or even an in/out office tray.
I hadn't taken instructable photos of the original on a lady's bike, which has the blue base of the cat basket. (So these mostly just show the cream coloured up-turned top of the cat carrier.)
If your bike has a cross bar, you will notice that when you have a high load, you can't really throw your leg over, so you'll have to do a high step-over the bar.
In our case, we've got large woven bags as shown on the photos.
On arrival at the shop, just grab the bag, shop, fill bag and on return to bike, replace on cat-basket and leave.
Check how much weight your rack can cope with. The one on these photos carries up to just 25 kgs, (55 pounds) so not a weekly shopping for an entire family.
Ordinarily, no need to tie it down either. The blue base shown is 3 years old and no load has ever had to be tied down despite the wind or Belgium's cobbled roads!
But you'll still have to see other people having to unload their bags to place their items one at the time into their bike panniers/side bags.
Step 1: Get a cheap cat basket... or something similar
Urm.. obviously...a bike with a rear rack / carrier
Half a cat carrier-basket, prefably the base (bottom bit) which will be uniformly flat.
Cat carriers / baskets are also sometimes called cat crates, or cat kennels, or catboxes. They are used to transport cats and other small animals without them escaping. I do not mean the cushioned sleeping baskets.
4 biggish round-headed bolts (& matching nuts),
8 wide washers,
4 off-cuts from a roll of perforated metal
4 Spring washers,
2 small narrow bolts, + washers to fit,
2 small spring washers
2 wing nuts.
A Drill with a small drill bit and a larger one.
A biro / pencil/ marker of some kind to pin point where to drill the holes.
I used a vice-grips to snag the metal strips to make it easy to break off.
A round headed spanner (size 13 in this instance) the open ended spanner will do, but a bit fiddly due to limitation of space.
- avoid nuts, bolts and washers made of iron as these may rust, which would be bad for the bike.
It's on a bike rack, so it'll not be going anywhere, I guess.
First I unrolled a strip of the metal strip and cut it to around about the length of the bike's rear rack.. Using a vice grip to snag the metal strip and then bend up and down to breaking point..
When I had 4 equal lengths (with holes in line) I paired them off and using a small bolt, washer, spring washer and wing nut, joined each pair at one end.
I just tightened up the wing nuts by hand.
I then slipped these pairs on to the rack so that one strip per pair was above the rack and the other beneath.