Installing the wing tube in a pattern plane is not what was intended for this thread. The idea was an ARF, however I thought the ARC's came with the wing tubes already installed.

Regardless this allows you to get the fit perfect and dead on. I prefer this usually. The Focus Sport I built was really close, but I think I could have done a little better by my own install.
So a quick and dirty review.

The sloppy fit is on purpose so you can shim and adjust to the wing in the model straight.

On a normal pattern model I like to work from the fin to the stab to the Wing. This keeps things in line easier as the fuse is molded. In the case of the Focus models the Fin is not always guaranteed to be straight as you have to glue it on. So working backwards from wing to stab to fin is probably the best course of action.

The plywood rings are easy to make. Just get a set of hole saws from Harbor Freight. The plywood centers that you cut out with the hole saw are the blanks. Use say a 1.5" hole saw maybe 1.75" then a 7/8" hole in the middle of the donut.

You triangle measure the tips of the wings on the tube and tube in the fuse. You also check the roll axis for straight. Shim and block up things to get it all aligned properly. Then tack glue the ply rings in place and on the fiberglass tube with a couple small dots of medium CA. Then carefully remove the wing and tube and use 30min epoxy to paint the donuts and build a small fillet between the fuse side and the wing tube center section.

Sometimes you have to elongate the holes in the fuse a little more to get everything straight. Most important is the alignment not the perfect fit of the socket into the fuse...The ply donuts are for that....make them fit the fiberglass perfect slip fit. Then the ring gets glued to the fuse sides.

I'll get in touch with the guys and see if the ARC models have the tubes installed already or if its standard to let the builder do it.

Either way its not much work, a couple hours tops. This above was a fast and dirty explanation to do it. There are other resources with step by step instructions. I just don't have them handy right now.

Troy