I've done dry-hops twice in a carboy. I don't use bottle shaped vessels of any kind anymore. I use kegs, and I generally dry-hop directly in the keg.
The first time, I did them loose---just poured them through the opening and let them float in the wort. Not recommended. It was a nightmare. They float on top of the wort and potentially increase O2 uptake. They clog the siphon/racking cane when you try and transfer.
The second time, I put the hops in a hop sack with some sanitized glass marbles. the marbles keep the hops submerged---better wort contact, and less change for excess O2 uptake. This worked fine. Its a bit of a challenge to get the sacks back out of the carboy, but it just takes a little wriggling and some patience.
I've transitioned to using lady's knee high stockings. I will use multiple stockings one for each charge. I would think these would work well for carboys, as the stocking is already in a narrow tubular shape. I soak the stocking in star-san for a few minutes, then I remove and drain the excess sanitizer. then I add the hop charge and 5-10 sanitized glass marbles, and tie the opening in a knot. the glass marbles weigh the sock down and keep it submerged.
According to Vinnie Cilurzo (Owner/Brewer at Russian River, create of Pliny the Elder and Younger) at 68F, 12-14 days seems to be the optimal dry-hopping length. After that the character begins to degrade. For Pliny the Younger, he has multiple consecutive dry-hop charges each removed and replaced after 12-14 days. I haven't experimented with this much, most other authorities report similar results, so I pretty much have taken that advice on face value. Lower temperatures will result in different flavors and a slower infusion versus the 68F temperature. Most every authority I'm aware of, seems to think that 68F results in the best character.