https://www.thingiverse.com/thing:4441076

There are rectangular holes for the nuts to be captured in. They are designed to fit snug, but not tight (aka not fall out, but not required a hammer to install). You will reuse the original limit switch, and it's 2 bolts. You will also have to remove the top gantry brace, and the entire X axis gantry to get this to slide down the outer edge of the rail. The upper and lower mounts are designed to slide freely in the T slot rail.

Parts list:
Printed
1 - Upper Mount
1 - Lower Mount
1 - Finger Nut (Optional)

Hardware
1 - M4 x 50 hex head bolt, fully threaded. (I ended up with a stainless phillips head that I ran a nut up to the head and locked in place)
1 - M4 nut ( plus one extra per above)
1 - M4 washer
2 - M3 x 20 Socket head cap screw
2 - M3 nuts
1 - Ender 3 original bed spring (if you haven't replaced them with yellow springs, you really should)

22/10/20 - Added versions for use with standard T nuts per request.

Creality

Ender 3

No

No

200

30