Hi.
I recently completed just what you are planning to do. I got 2 NOS feed
throughs last year on Ebay. They each consisted of a threaded rod with
nuts and washers, two cone shaped insulators and more than enough
ceramic cylinders, for the rod, to pass through my wall. I had to cut
new cork gaskets to go under the insulators. The old ones just fell
apart.

I had to construct a similar feed through for the connection to the
ground just outside my shack. Here I used a threaded rod, two 1/4 in.
plastic disks, and a plastic tube cut to length for the wall feed
through.

If you are anywhere near a plastic supplier, such at TAP plastic, they
will have all the stuff you need.

Good luck.

Paul, KD7HB