If your front baffle is 5/8", then you want port area of 15.9in2. I'd go with three slots 1 1/2" x 3 1/2".If the baffle is 3/4" then make port area 16.8in2. Use 1 5/8" x 3 1/2" slots.
If 1" baffle, then port should be 17.7in2. Make the slots 1 5/8" x 3 5/8".