PFS question, not HL-specific

[Azhrei]

There are a lot of people here who have more experience with the various Paizo source books than I have, so I beg your help.

In a game a couple days ago, two rogues in our party were invisible and moved to surround an enemy mage and take them out. The GM said my attack would be a regular attack, albeit with the benefits of being invisible, but that it wouldn't be a coup de grace since the target wasn't "helpless". That's fine, but it meant the three rounds spent getting into position were rather wasted since the rogue could've been more proactive against other combatants.

So here's my plan for next time. I will buy three small sacks with leather straps used to cinch the openings. While invisible, I'll pull the sack over the head of the target from behind them. It would be easy enough for the target to pull the sack off, so I want to put some kind of spring-loaded "fishing reel" on the leather strap that will suck up all of the slack. In an ideal world, it would work like a wind-up clock, where a key is used to coil the spring by turning it clockwise, and inserting the key and turning counterclockwise would release the ratchet. The device would be attached to the inside of the sack so that the target couldn't get to it easily.

Here's my problem: we're playing by PFS rules, so I cannot craft anything custom. Does anyone know of a device similar to the spring-loaded reel that I described that already exists in Golarion that would be acceptable under PFS rules? I know the concept exists, because of the spring-loaded wrist sheath, I just need something that will spool string or twine or thin leather strips...

Thanks in advance!