Hi Seasidesunflower,
Somehow I am astonished that nobody answered your queston till today. Although not a banjo player myself but may be having an idea of what you moves to make your beloved husband a banjo strap, I was googling on the internet to find some help for you. Myself, I don't have any patterns for a banjo strap/banjo cradle strap, but may be the following internet sites may assist you in getting you to your target. The 1st site is: Pete Schmidt Artisan Made Leather Banjo Strap, 2 parts, on YouTube, shows how such a strap looks and is mounted on the banjo, sadly without patterns. The 2nd is: www.ehow.co.uk/how_5955636__make-banjo-straps.html; or just: How to make Banjo-Straps - eHow by Matt McKay in Hobbies. Googling further yourself with "How to make a banjo strap/banjo cradle strap?" may bring you up even further results of which one or an other could help. A final suggestion for now is: go to a music shop, preferably one specialised for string instruments like banjos, guitars, mandolines and so on and possibly buy a low priced banjo strap there, check it closely how it is made, take all the necessary measures from this strap and maybe of the parts of your husband that are critically for the size/length of the strap and make a pattern accordingly. With this words I hope I can be of some help to youessi,and it is desirable that other members that have more knowledge of such a project will chime in, soon.
All the best for you and God's Bessings for 2022.
Markus L.A. Drescher, "The Leatherthresher*, Switzerland.