I think Edwin's link with the top mounted bearing is the winner but Thanks
I an picturing framing an old floor furnace opening that will be used as a
return air grill for a new HVAC system. Someone botched the job years ago
and I have lived, actually my tenants have, with an unsightly section of 2"
scraps for the last 25 years or so.
I prefer to leave the tongue on to insure a tighter fit and to minimize