That's funny. I'm doing the same thing right now. 15 gal in my Sanke fermenter in a 7.2 cu ft Magic Chef. I made it as a keezer first, but didn't get a second one until a couple days ago, so my batch is fermenting in my keezer until I build another collar for the second one. I'm also going to get a third for my secondary.
On my fermenter everything is accessed on the top (bev/gas/thermowell,) except the bottom dump which I put on a 90 degree tri-clamp elbow. A butterfly valve is after that. This way it points out to the user when I want to dump yeast.
The most important thing, I think, for doing a chest freezer ferm chamber is that you MUST build one with the collar attached to the lid- so you can lift heavy vessels into it reasonably, in stead of over a collar.
I actually put my whole chest freezer on casters so I never have to lift a filled fermenter. I fill it at the brewhouse and then roll the whole chamber over to its place. Makes shaking it (and the starter) around to degas, etc., very easy too. Just get good casters (castercity.com). You can't do that with an upright (well, you could but..).
I'm so glad I chose not to do the upright freezer for 'my' specific ferm vessels. It is much more low profile, yet holds a ton of beer. My 15 gal secondary fits into it with room still for 4 5 gal cornys. Thats 35 gal of beer in a rollable package. Tight. I will have to make my collar a bit higher for that one (1 X 10, or so)
If you are going to do it this way, then here's a tip I think worth following:
If you make the collar out of 1 X 8 tongue and groove board, and then cut a 45 degree angles at the corners (like a picture frame) the grooves and tongues will line up perfectly so that you can just (if you measured right and cut well with a miter saw) wedge the tongue into the groove that the rubber seal goes in-- and then the rubber seal wedges into the groove on the collar (as seen in the pic). No glue at all. I just put a couple screws in to make sure, but it really holds tight by itself. And, I did nothing to the freezer that makes hard to convert back to just a freezer, should I want to sell it and get a different one .... for some reason (prob not going to happen...)
As you can see in the pic, I just bolted rigid pink insulation on the inside. The thing has incredible r-value and doesn't run the compressor much. Since it's such a small package and fits right around my fermenter like a glove, I can cold crash super easy/efficiently. It's almost like a jacketed fermenter the big boys use, at least in my mind
Edit: Guess I didn't mention that the 3 gal corny is the starter vessel and the yeast harvester.