I saw a review of a dry hopper on Brulosophy that attaches to the top of a stainless conical and has a butterfly valve and a chamber that can be purged with CO2 before dropping the dry hops into the beer. Nice looking hardware, but it costs over $200 !!! I began to brainstorm how to something similar on my setup for a lot less money. I ferment in a Fermonster, a PET fermenter with a 4" plastic screw-on lid with an O-ring seal. I have been dry hopping with mesh bags or stainless mesh tubes that I hang in the beer with dental floss. I know that the lid will seal tightly even with dental floss going under the O-ring, and I can then pull out the hops when I am finished. Up to now I have been removing the lid to add or remove the hops, letting some oxygen in.
The next time I dry hop I am going to put the hops in a mesh bag (with some stainless plumbing fittings for weight) and use dental floss to hold the bag up just under the lid, out of the wort, at the start of fermentation. When the time comes to add the dry hops I will loosen the lid ever so slightly and let the weight pull the hops down into the beer, then tighten the lid again. After a few days I can loosen the lid again and pull the hops up out of the beer but leave them hanging until after the beer is transferred out. This should allow for dry hopping with no oxygen exposure.
I haven't tried this with real beer and hops yet but I have made test runs with an empty fermenter and it seems to work just fine. And it's free!