I have an older 2136 Horn cabinet. Horn still has it listed with the outside dimensions for the cutout. I have a Pfaff, and had an Ellure. Had inserts for both.
I also have a Koala Lite cabinet. The original insert for the Pfaff was made for Koala, it fits the Horn exactly, and is now on the Koala Lite.
I just went through all this with the Brother 1500. Those cheaper inserts are made for the adjustable tables like the Arrow and the other table that is very well liked. The inside dimensions of the cutout DID NOT FIT either the Horn or Koala with the lift.
This is the standard cutout size from the Horn site.
18-13/16" X 11-3/16" cutout for sewing machines
They also have a lip.
The newer tables do have larger cutouts for the newer, bigger machines, but sounds like you have the traditional one.