I have tried... it seems like the o-ring fastening points are not in the right place to get the best leverage. I found a spring that should be strong enough and the right length but the angle it has to pull at - it can't lift the lever, just take some of the weight off of it. I plan to give it another go sometime - look into a different mounting point on the fixed piece - but for now, like mentioned above, it works fine without o-ring or spring.
I swapped the two attachment/adjuster bolts to get a better angle + longer distance, plus I put one twist in the band. Works fine, but I agree that more tension would be better.
BK