$10 and $15 a crack... are you nutz? $30/pair comes out to more than I paid for a cam cleat. It seems to me there's a dummy tax applied.
If it we me, I would pool the money you were about to spend, and hire someone to spend a few hours and help you sort out your lines. Agree beforehand to abide by his recommendations for, say, two months before discarding them for a store-bought doodad.
Could the problem be something as simple as using the wrong kind of rope? I can see how a stiff, braided line can slip a clove hitch, but the soft-handed lines I've seen on all fenders snug up very securely on lifelines. A clove hitch is easy and fast to tie, even more so if you tuck a loop rather than the entire end. Sung it up tight so it bites into itself and won't go anywhere; the loop makes it easy to undo (just like your shoelace).