Mount opening is 1.6 inches inside diameter. I redesigned this with a thicker mount bracket than what is pictured as my original was a little flimsy. I recommend this with an LED bulb only. If this design gets enough interest I'll update it with other standard mount sizes. 

Print Instruction: Print top side (bracket side) down. If you run into first layer issues use a raft or brim.

Specifications: Dimensions are 8 inches wide by 7.25 inches tall. 1.6 inch I.D. Mount opening. 

If this works for you please post a photo, I'd like to see the improved bracket and know if it is sufficient. Thanks!