It's a Big Mouth with a spigot. I get too much hop debris when I commando, but when I bag the aroma and flavor suffers.
Use a bigger bag.....a 5 gallon paint strainer bag or similar bag made for home brewing, crush the hop pellets, and agitate the fermentor a couple times during the dry hop period.
You may still have the bag get stuck in the spigot when racking though....unless you can tie it off to the lid somehow.
Also...preventing suck back during agitation is hard if you don't have a way to provide a little head pressure with co2.
Now that I think about it....no wonder people don't like bagging their hops....the most ideal fermentor or secondary is a keg which you can shake with head pressure while dry hopping within a suspended bag.
Not sure I helped.