Mounting gear on your will be somewhat limited, especially with no tail section. Obvious as thats why you are asking the question. giggle. One option that I use, would be a tank bag of your choosing. I happen to already have a horn bag used for a horse/mule saddle, that I also use on my TW. I am able to put it around my gas cap, which keeps it secure. Mine is a bit large, but I have had no problems with any contact or cooling issues with the engine, and plan to continue to use this tank bag for most of my outings. Holds quite a bit of stuff and has 2 water bottle slots. Course, there are many many types of tank bags to choose from and they sure would come in handy in your limited cargo capability.
Another thought is a backpack. I and probably many other riders have used a backpack both for street and off-road use. This winter I got a bird hunters suspender pack. Not really sure what to call it. I wanted something slightly different than the average backpack. The front part has 2-3 inch suspenders which have a main belt type buckle and a chest strap buckle to hold the suspenders from falling off the shoulders. Attached down around the buckle on each side, it has various storage compartments and one with lined pockets for your hands. These storage compartments are originally used for ammo and suff like that, but I use them for other small things.
The back part of the vest is for putting in birds, and is sort of like a back pack. I use it to put in various larger items and water. If it gets hot out, I can stuff my jacket in the back or vice versa. Also the rear compartment is expandable. I haven't used it riding on the TW, but I took a 5 mile hike with it a couple of days ago and am very please with the outcome.
If I can find a photo on-line of the product, I will post it here for your reference.
Also was wondering if you have enough room to attached those Agri-storage tubes Truelight and others have attached to various parts of the TW.
Anyway, just a couple of options I use because my dog rides on back of the TW and takes up all my cargo area.
I chose this one to keep the vest contact area to my body minimized so I wouldn't overheat or sweat while wearing it.