How to secure backpack on rear rack?

Hello fellow commuters.
I saved a rear rack from going to landfill. I have tons of old backpacks that I can re-use, I don't want to go out and buy a pannier bag, if backpacks can be fastened to the rack securely.
Surely there's a hack to secure ordinary 20-25L backpack on top of the rack?
Quick google search didn't really give anything convincing. There was a wrap-around-with-bungee-cord hack, but it's very wobbly. There was also a hack to prop the bag upright at the back of the saddle and hold it down with the spring clamp (the rack has the clamp), also unstable.
Any hack / tips to securely fasten a backpack on a rear rack?
Or just get a pannier?
I saved a rear rack from going to landfill. I have tons of old backpacks that I can re-use, I don't want to go out and buy a pannier bag, if backpacks can be fastened to the rack securely.
Surely there's a hack to secure ordinary 20-25L backpack on top of the rack?
Quick google search didn't really give anything convincing. There was a wrap-around-with-bungee-cord hack, but it's very wobbly. There was also a hack to prop the bag upright at the back of the saddle and hold it down with the spring clamp (the rack has the clamp), also unstable.
Any hack / tips to securely fasten a backpack on a rear rack?
Or just get a pannier?
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https://carradice.co.uk/shop/saddlebags/super-c-seatpost-qr-slim-saddlebag/
or https://carradice.co.uk/shop/rackpacks/super-c-rackbag/
I've used the top SQR bag for the last 3yrs or so with no issues and still looks pretty new.
Seriously though, I would just buy a pannier - Lidl and Aldi sell them at various intervals, and Planet X certainly used to sell some very cheap ones.
Just think about the ramifications should said rucksack slip and end up in your spokes.
You might get away with it, but equally you and or your bike may end up with major damage.
Is it worth the risk for perhaps £30?
How about these - a pair for £47, although free postage is only on orders over £75.
I've ordered from them several times, and there were no extra fees for me.
https://alpinetrek.co.uk/stoic-granvikst-back-pannier-22-panniers/?aid=7938098ff4b968084c573d44e724e5ab&pid=10004&gclid=CjwKCAjw8JKbBhBYEiwAs3sxNwYsIIA5821QGCsabQWElj3EHpYmGLzX-x4FeW6wR9prSxa2TajBchoCJQ4QAvD_BwE&wt_mc=uk.pla.google_uk.18006637156..
They look like a bit of an Ortlieb clone to me, and are classed as fully waterproof too.
Edit: Single one is £25.98, and it's only £3.99 delivery:
https://alpinetrek.co.uk/stoic-granvikst-back-pannier-22-panniers/?aid=7938098ff4b968084c573d44e724e5ab&pid=10004&gclid=CjwKCAjw8JKbBhBYEiwAs3sxN8ywrD6qNNkUQIT4fy51LnYS-rneqO84YIoRAGnc7VuvOVOQseXFXRoC5U8QAvD_BwE&wt_mc=uk.pla.google_uk.18006637156..
Marin Palisades Trail 91 and 06
Scott CR1 SL 12
Cannondale Synapse Adventure 15 & 16 Di2
Scott Foil 18
A more permanent fixture could be made by inserting a thin plywood stiffener sized to fill the back of the pack. You can then drill this and use suitable nuts and bolts to mount to p-clips on the bike frame or some lengths of thin steel or alu strips to clamp the bag against the frame and rack.
I'll save a littered basket, prevent unnecessary purchase of new pannier, double ticks on saving the environment.
I get that pannier is the straightforward answer. I'm just so hung up on spending more money on something new that I potentially don't need by re-using things that I already have. I'm on a look out for a pannier on local freecycle, though.
I also agree with the risk of having straps and bits flapping about above the rear wheel. If a bag is contained (like in a basket...) then the risk is greatly reduced.
Being a commuter, I'm not fussed about the look or being aero. Function over appearance.
I am not sure. You have no chance.
Marin Palisades Trail 91 and 06
Scott CR1 SL 12
Cannondale Synapse Adventure 15 & 16 Di2
Scott Foil 18