Pond garden centers have bulkheads for plumbing koi tubs. but most of them are closed for winter yet. come to think of it , i have spare bulkheads from 1/2 inch to 1 1/2 inch. pick a size and i'll give you one. I got a bunch cheap online. I would go with the 1 1/2 inch sink drain size. elevate the fuge a little so that the bulkhead overflow can be piped with an elbow 90 to go over the top of the sump tank and dump. that way flow rate will not be a problem. I am upgrading my overflow lines on the plexi tanks to 1 1/2 inch and they now handle 600-700 gph easily.
I found that the 1 inch bulkhead and pipe could only handle about 200- 250 gph when used as a surface skimmer overflow drilled at the water line. so that a maxi jet would push it at it's limit when used as a sump pump to feed the fuge up a couple feet.
If you want, i got a great rectangular, sturdy, plastic storage box. about the size of a 10 gallon tank. I was gonna use it for a fuge too. like $5 from pathmart. nice flat sides, pretty thick plastic. would take the bulkhead easy. actual dimensions are 18 long by 12 wide and 13 inches high with a little taper. so actually its about 7 gallons. nice size. with a lid! would be perfect.