Hi Rick
I used a 3" hole saw for mine and a mouse pad as the flapper and it works well.
I'm not near my Keene right now or I would meas what they did.
Mike
Beach High Banker, Sweep Jig, Whippet Dry Washer, Lobo ST, 1/2 width 2 tray Gold Cube, numerous pans, rocker box, and home made fluid bed and stream sluices.
The hole does not need to be very big but flapper must overlap the hole enough on all sides to get a seal on the upstroke then fly up a bit on the downstroke to allow air to immediately enter. I used vinyl upholstery fabric for the flapper on a mini dry washer I built. I think the hole on my Whippet is about 1 to 2".