Our unassembled 4-frame bottom board is handmade in the USA, using 3/4" tongue-and-grooved pine with 2" x 3/4" dadoed rails.  The pine is commercial grade wood. Any knots or other imperfections in the wood will be small and tight.
 
The bottom board is reversible, giving you the choice of a 1/2" or 3/4" entrance.