I'm certainly no expert, and I love bowlines as much as the next guy. One of the reasons I love them is because, as sturdy and reliable as they are when under load, they're equally easy to untie when NOT under load. That quality would worry me when used to secure something where the load goes from strong to none repeatedly. The other posters mentioned using a bowline with a stopper knot or some other backup.. that'd probably be fine.
I like buntline hitches a lot; they don't release under no-load situations, and get more secure the more the load increases.
Buntline Hitch - How to tie a Buntline Hitch
They can be tough to untie, but not terrible. I've taken to taking a loop through as the final step when tying it, rather than the entire working end; it becomes sort of a "slip buntline" that can be untied with a single pull, but still seems just as secure as a regular buntline. That's what I use on my fenders now.
Just an idea. I'm really new, so don't take my advice without other research and experimentation.