Have I inspired you to start constructing a capo-ble banjo? Even if not, you might be interested in what’s involved.
The top-of-the-list book on banjo building is Construction of a 5-string Banjo: a Technical Guide by Roger H. Siminoff. It was my reference when I built my banjos.
(Please note: Mine is not a commercial site. I don’t sell anything here, nor receive any compensation for my personal recommendations.)
If you dip into Siminoff, you’ll quickly discover that building a neck and building a pot are radically different enterprises. A moderately skilled and patient woodworker can make an excellent neck, and might not need to buy one additional tool for the job. A standard pot, on the other hand, is a precision industrial product. Siminoff tells you how to do it, but he scared me off.
It’s not difficult to alter Siminoff’s full-scale plans to suit your needs. Just bear this restriction in mind: For G or C tuning, the strings cannot be longer than 22”. A full length banjo with 25” strings must be tuned lower—to the E or A equivalent of G or C. (That has an interesting mellow sound. And you can, of course, capo up.)
You can buy all the parts except one needed for a capo-ble banjo neck from Stewart MacDonald, the string instrument supply store: https://www.stewmac.com/?utm_source=bing&utm_medium=cpc&utm_campaign=TXT%253A%2520XStewMac%2520Brand&utm_term=stewart%2520macdonald&utm_content=StewMac%2520Brand. The one thing you can’t buy there is the neck wood. The unfinished neck blanks they sell are carved specifically for standard 5-string banjos. You’ll have to buy a piece of maple or luan and carve it yourself.
You can also buy a complete pot kit at Stewart MacDonald, everything exactly machined. The price may give you pause, though. Unless you want to construct the most elegant capo-ble banjo possible, buy an inexpensive banjo for its pot, or look for a second hand one.