Koplin makes a nice one that attaches to the back rack and rests at your foot, or floorboard in my case. Boot is lined. Great for keeping mud off your gun. My 56 mm scope atop my gun fits with no problem.
Mine has a quick release that allows the boot & bracket to come off my bike. It leaves only a flat plate attached to the rack. It even has 2 positions that allows the boot to be tucked in close to the bike or kicked out about 10" to allow more room near the floorboard.
The only negative is the top section rattles A LOT. The fit between the top & bottom is good but they make a ton of noise rattling against each other. I may try some foam tape between the 2 sections to see if that quites them down some.
...I got rid of the plastic mounting/quick detach thingy and just strapped it directly to the outside of the rack (Polaris). Doesn't take up ANY rear rack space, is completely out of the way of the floorboard, and doesn't rattle/vibrate.
Its the top section & the retaining clip rattling around that makes all the noise. The quick release attachment does not make any noise, if its tight enough. When you remove the boot it leaves a 3" x 6" mounting plate attached to the rear rack. The plate does not inter fer with mounting anything on the rear rack.
Some days I leave the top section off and just slip my rifle into the boot. I usually put my Marlin 1894P .44mag in the front bag & my scoped rifle in the boot. That way if I want to stand hunt I'll use the scoped 7mm-08 and use the .44 lever gun for slipping around through the thick stuff.
Either way the boot is a nice way to tote your rifle around if its wet & muddy out.