I created this bracket to hang an Ikea Kadrilj blind from the curtain rail base which saved making new holes in the wall.  The Ikea metal brackets screw into this 3d printed bracket.  I suggest using brass heat set inserts to bolt into this bracket.  I sized the holes to fit the heat set inserts which can be inserted using a soldering iron.

I printed these brackets with eSUN PLA+ in cool white so it matched the curtain rail.

I suggest printing the bracket on its side for maximum strength.