around here, there are several ranchers with scabbarded rifles on their tractors & ATVs for pest control of the dog packs we've got. For some reason, folks seem to think that dumping that litter of rottie crosses out in the country is a good idea once they figure out the puppies will grow into dogs too big to keep in their townhouse/apartment. Unfortunately, if they live to adulthood, they're very efficient at taking down the cattle the folks out in the country are trying to raise to feed the city people... Couple years back a neighbor potted 3 of 5 that were attacking one of his herds -- he only got 3 because they were the ones who came after him. The other 2 ran the other direction.
the nice thing about that gun boot (other than the fact their price has come down over the past couple of years) is that if completely encases the rifle to protect it from brush, dirt, and the weather, plus most of that style are lockable for secure transit if you ever travel with your long arms.
the bad part about it is it completely encases the rifle making it slower on the draw.
given that, I'd lean towards something along those lines if you're looking to buy a finished product. (you can always leave the top part off and have a good scabbard) You could mount it on your "strong side" fender so it's ready to hand and it'd probably do you well.
If you were going to make something yourself, I'd buy a $3 gun sock (or sew one up out of an old blanket) and make a scabbard out of HDPE (plastic barrel) Plastic barrels are cheap and cut easy with sabre saw or rotozip. (they also make great sleds!) - and you can soften the plastic with a heat gun to form your scabbard around the rifle in the sock to fit it. (The sock protects the rifle from dirt in the scabbard and keeps it from rattling) When finished, it'll look sort of like an old leather scabbard - only thicker and blue (or whatever color your barrels run up there)