Trunk Storage Bag & Mercedes Medical Kit
 

I was going to buy a custom bag but then decided I'm wasting money. The ones out there all seem to use variants of velcro to stop sliding.

Used a small six compartment Wegmans wine bottle soft bag, for my rags and internal/external cleaners. That is attached with one powerful bar magnet inside to the ring on either side of the trunk inside cover. Another two bar magnets attach the Wegmans bag to the Mercedes Medical kit.
It's all black and pretty much out of the way on the left side so it looks aesthetic (you barely see it and the trunk cover comes off no problem).

I put the medical kit under the trunk area (not sure what it's called). It doesn't exactly look like it was designed for random storage but there's a spot where the bag fits.