Most router tables come with a plastic insert with various hole sizes to accommodate different router bits. This router table was a garage sale find and was missing the insert. With a few quick measurements using dial calipers, I sketched up a quick model here in Onshape.

https://cad.onshape.com/documents/134deb37bf785e4c1a9b1d6d/w/a0b1020318c6ae098f493642/e/43419f4f13a7b02609b8f29d

The design is fully parametrically defined. These dimensions have a 1.5" opening and worked great on my Bosch RA1181 Router Table.