Chainstay Bridges for Fenders?

Hello all, I am building a new winter bicycle for myself. I want to add a chainstay bridge to attach a fender nut to, so that mounting will be easier. Are you guys buying them prefabbed? If so, from where? I was thinking about just making my own but needed recommendation on tubing sizes and where to get it.
Thank you - Todd.

1/2" .035 wall 4130 from McMaster Carr

use the 14mm cutoff from the top of a seatstay

install a bottle boss in it facing the hub that can be used for a through-bolt on the fender.

Thank you guys! I appreciate the speedy responses!