You're solo......you want the two saddlebags, the trunk, AND this bag? That's a lot of stuff.........how are you going to open the trunk with this on the back seat?
I'd try packing smarter, lose the bag, or not buy the trunk.
I don't agree with bungee cords. Bungee cord hooks can pop off, hit you in the eye or go into the rear wheel. They also flex; which makes the wiggle and or fall off.
I use ratchet tie-down straps. I put the hook over the the saddlebag quick release knob on each side and loop the strap around the back. Each bike is a bit different where the straps will go. I also sometimes use a loop of 3/8" nylon rope in some places too.
When my wife and I are traveling we do both motel and camp, so we have lots of gear. The tent, pads and sleeping bags are on top of the trunk. The trunk is over half empty, so if we want to put the jackets in it there is room. She gets the right bag and I get the left.
When traveling alone (no wife) sometime I use my dry bag on the seat, sometimes just put my lightweight sleeping bag in the trunk.
You only need to bring 3 or 4 changes of clothes, no matter how many days you are traveling. Try to bring an extra of pants that are not jeans (some lightweight type of fabric).