OK. To save you all the trouble, I just removed both rear side pods. Neither has enough room to store my kit and from what I saw, the BMW kit must be more like a bicycle repair kit in size. I reckon, if push came to shove I could split the kit and store a bit in each side pod and a spare CO canister either side of the TPMS/Alarm unit under the seat.
A suitably small tool roll would be easy to bungee under the tail light and not get in the way of the number plate light.
And, if by 'panniers' you mean Sport Cases, then a longer tool roll would be easy to bungee/tie-wrap to one of the Sport Case mounting rails. Near-side would be my best bet as it would be less visible when the bike was on its side-stand.