Your rollup bag and straps will work. I prefer the saddlebags since they keep weight low and are not in the way of movement but nothing wrong with the rollup bag in my opinion. Just strap it tight.
So I have a few setups depending on the ride. It may or may not be cheap.

To me spending R3000 on the Giant Loop Mojavi bag felt expensive at the time, but I freaking love it and would spend it again - easy. It;s out of the way doesn't move and is hardy.
So anyway. I use a tankbag and the Mojavi Giant Loop bags on basically any and every ride. The tankbag is basically for some snacks, store phone, and miscl things. Take off shades (i fit shades under my visor mostly) / goggles throw in there etc. If it;s a local ride it holds water as well, 500 - 750ml. Or I take a water bladder.
Then in the Giant Loop Mojavi I have Medical aid kit, Hand Pump, patch kit, Tools, Battery jumper, cable ties and Pratly steel putty and masking tape.
If I go further i have one big drybag (Togbag) that I can put Sleeping bag, tent, and stuff in and roll up. I strap that with Rokstraps. Tent on top, Petrol bag ontop of that.
Backpack with water and clothes. (I put the water bladder inside the backpack.)
Mate of mine has the Wolfmann saddle bags. Seem to hold a bit more. Shorter but wider than the Mojavi's
None of those require nothing like plates or scaffolding on your bike.