I've gotten Deering Goodtime co-rods from them -- even though they're not listed under Parts. I just called, got a price, and placed the order. They've got a standard thread to connect onto a neck hanger bolt.

I use a variation on dowel sticks. The Co-rod and dowels are keeping the string tension on the neck from busting the rim. I use a dowel that is glued in place. The neck uses one or two lags and nuts to mount to the rim.
Usually use a single dowel but clearance to the lag can be an issue.
Have never had an issue with this set up. And it makes neck removal simple.

Tom, this looks interesting - so your dowels don't run into the neck heel at all then? You use hanger bolts to attach the neck to the rim, and then have dowels running through the rim just to provide some additional structural support? How do you attach the dowels, and do they run through the rim at all or are they sized just to fit snugly from wall to wall, and then secured with screws from the outside in?

Actually, I can see now in the photo with two dowels that you have them running all the way through the wall of the rim. How do you secure them inside, wood glue? And do they run through the tailpiece end the same way?

Link to Ebay co-rods

I added a second simple rod of threaded rod stock from ACE hardware, to my goodtime rim. It was warping from years of string tension, and the existing single rod just exacerbated the warp when trying to adjust it. With the second rod, I now have a perfectly round and adjustable rim.

I don’t see any reason you couldn’t use threaded rod to support your rim, and either bolt the neck on, through separate mount points, or even use threaded rod as co-rods, using coupling nuts to the threaded studs (hanger bolts). hanger bolts are standard hardware, and available at ACE as well.

I use Titiebond II. Since the dowels are under tension, a small diameter( 1/4" to 5/16") oak dowel is fine. Use a forstner bit to drill.
Dean's suggestion of a threaded rod works well, too. Same structural support.

You could also use a flat stick. I did one with a carbon fiber stick about 1/8" thick x 1/2" wide, screwed to the bottom of the pot.

Thanks for the suggestions. RioStat , I saw that listing but those rods are 9 3/4", so they're a bit short. I'll look into the dowel sticks and threaded rod from ACE options. I just want to be sure that I have a reliable and solid method of attaching the neck because the components I've collected for this banjo are really nice, and I want the full instrument to live up to its parts. I'd be upset if the neck just didn't join correctly or if the rim warped.

I built another resonator back earlier this year that I love, but it's imperfect - it's got a 12" rim and I secured the neck with two hanger bolts, and then ran a 3/4" dowel from one bolt through the rim's body to the other end, where it's secured with a tailpiece screw. It works, but between the 12" rim and the somewhat jerry rigged neck connection/dowel construction, the tuning can be a bit wonky. I'm trying to avoid that this time around.