I needed a mount to attach the brake light I got off of Amazon: https://www.amazon.com/dp/B0B4JCX6YN?psc=1&ref=ppx_yo2ov_dt_b_product_details

(Victoper Bike Light, High Lumens Super Bright Bicycle Light, 6+4 Modes USB Rechargeable Bike Headlight & Tail Light Set)

The rails measured about 50mm apart outside, 35mm apart inside.

This light mount attaches to the saddle using (2) M4x18mm bolts and (2) M4 Nuts. I recommend printing 100% infill,  with supports everywhere.