Topboxes and their mountings have improved over the years but, as noted above, they bounce the most. Anything valuable in there, consider a ratchet strap around the whole thing and under the mount. Otherwise tankbag. I use the small Wolfman tankbag, Giant Loop Coyote soft bag, (Enduristan Rally Pack tailbag and Wolfman fender bag (puncture repair stuff). Plenty similar alternatives now; all quite pricey.