The main part in this is the small adapter, it is just a friction fit onto the wiper (you may have to remove the swivelling part that is used by the car to attach to first).

The handle can either be printed or just made otherwise (I used some corian acrylic and put the round overs on with a router).

Two heat set inserts (M3) are melted in from the inside of the adapter, then the handle can slide in and get screwed in with two countersunk M3 screws.

Onshape Link: https://cad.onshape.com/documents/7b73736725ae1ff779056eeb/w/af8cacbfd430afa2e5615c6a/e/95fd527c1025daca53485e72