What, I hear you say, is this?
It started life as one leg of a pair of waterproof trousers then I slit it up the side so it was a flat piece of plastic material. Then using my ancient sewing machine I turned the edge and sewed a seam, then passed a long bit of string through the seam edge. Then my toolkit went into the space created.
So it's possible to tighten the string and the top of the "bag" comes together
And when in your saddlebag it would look like this:
Why bother? Because when you take your toolkit out you can just loosen the string and drop it on the ground and all your bits will be on display (as in the second picture) rather than rummaging around in the small bag one used to carry your kit.
I'd be the first to admit this solves the smallest problem imaginable, and I made a similar thing for my climbing kit and it worked well.