Although I don't have any experience holding down gas cans with Velcro, I would tend to go with the ratchet straps. You can snug them down easily and I suspect that the Velcro will not last long as it gets covered with dirt, mud,etc on the trail. My experience with the use of Velcro as adjustments on gloves, etc is that eventually it will not hold as it has picked up all kinds of debris that has stuck to both the "hooks" and the "fluffy" material. Stick with ratchet straps.
I too have a hard time drilling holes! I don't know if your box is the same as my previous '05 500HO but this is what I did. There were two steel rails mounted to the top of the box. I removed 2 bolts from each rail and made a plastic spacer for longer bolts. I try to use existing holes wherever possible.
I then mounted a short piece of aluminum flat bar, one on the left and one on the right going from the front rail to the back one using the plastic spacers (1/2" to 3/4" high bushings). I then mounted a Rotopax universal mounting plate to these two flat bars. The Rotopax fuel cans then mounted to the universal plate with Rotopax deluxe clamps. This was a very durable mount and was easily removed when I sold the bike. I still have the parts somewhere.
Do not use Velcro straps or any Type of Velcro to hold a fuel tank down. Strapping anything down to a Polaris rack is a PITA so I would bite the bullet and get a tank mount and drill the holes to mount it. You could also get the metal rail that mounts on top of the rack (i.e. more holes) and then strap you tank to it.
I've had two of the Kolpin 4 gallon fuel packs on our Sportsman 700's for 4 years with no problems. I have them strapped to the rack extensions on three sides with the velcro straps that came with them. They stay secure for the most part but I added a couple of stretch cords over the top to hold them down on rough trails otherwise they bounce around some.
Are you referring to velcro straps or velcro strips? If it were me I would just use a set of those cinch straps that have the push button to release. It would be a lot quicker than a ratchet strap and plenty strong enough.
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