Under the seat I have a Rough Enough bag zip-tied to the frame rails, hugging the back of the fender. Because it's zip-tied in, I can open and close the zipper without having to hold the bag or have it move about. This is the bag I bought: https://www.amazon.com/gp/product/B00HYTQL3U/
It's a lightweight cordura... and now I'm seeing this bag: https://www.amazon.com/Multi-Functio...dp/B00KDRH6Q6/
, which is a rubberized version. I will probably snag that, as it will protect its contents better.
Anyhow, inside that bag I have my motion pro belt tension gauge, a modified Kutmaster Wrench-Head multi tool, bits for the tool, a flashlight, a Leatherman Squirt ES4, a Swiss Tech adjustable wrench, a handful of zip ties, a lighter, a replacement clutch and brake lever, a microfiber towel, some wire nuts, a flattened roll of duct tape, and a pair of nitrile gloves.
I've attached pics of the multi tools below!
I like the Kutmaster Wrench Head because the lower handle has a bitdriver in the end, and it can open 90 degrees, turning the tool into a T-handle bitdriver. All of the blades are also locking, and the tools are nice and BIG. I got the Winchester version of the tool, and then also got a Wrench Head version of the tool. I swapped some blades around to get a setup with a can opener, saw, file with gasket scraper, clip-point blade, and spark plug gap gauge. It originally also had an awl blade in there, but I modified some scissors out of a cheap hammer multi tool, and replaced the awl with scissors. I also used the Wrench Head sheath, as it's far larger. Into that sheath goes the black bit holder (which comes with the Wrench Head), which I've loaded full of my own bit selection. I also have a little red strip of allen key bits I snagged off of Amazon, which fits in there as well. On the right the flashlight tucks into its loop, and the center pocket, normally reserved for a few small sockets, houses the Leatherman Squirt.
The Swiss Tech wrench is actually extremely useful. It's a small adjustable wrench, but it works very well, and it includes a couple of tiny blades and another bitdriver. It fits in a sheath made for a Victorinox 130mm Ranger knife, along with a cheap bit extension. This is useful if I need to make a quick adjustment and don't want to crouch down and fiddle with the side panels to get a wrench out. It's a neat little tool. The Swiss Tech T-Rex pliers are also super useful for wire cutting and small plier duty, but the Leatherman Squirt does that job better so it got put in the kit instead.
The Leatherman squirt is in case I need to do any kind of wiring repair. This is more a necessity on a vintage bike, but even so it gives me peace of mind to have, and it takes up very little space.