Thanks for the replies and tips. The screws I used are short stubby machine screws, not pointed screws, so I'm not worried about them puncturing the bag. Although at this point, that might be a good thing...the hops swelled a lot more than I thought they would and the bag looks pretty full. If I could I'd take them out of the bag altogether. Oh well.
I used pellets. For my last dry hop (I just remembered this was my second dry hop, not my first), I just threw the pellets in but that beer ended up with a LOT of sediment so I wanted to try dry hopping in a sack.
There's no way I could fish it out now anyway...it's swelled so much I don't know how I'm going to get it out of the carboy! I tried giving it a poke with a sanitized rod, but I've decided to not mess with it anymore and let it ride. I probably won't get the most out of the hops, but lesson learned for next time.
I'm planning on bottling next weekend, so the dry hop will have been two weeks...maybe they'll sink by then.