Use an overhead milling machine with a 1/4" bit.  Make bottom of 1/4" BB.  Round over leading edge of bottom wit sandpaper.

If no access to OH Milling machine use a router table with stops set up.  Slowly increase slot depth.

Pics seem to indicate bottom is solid sawn.  Could round over leading edge of bottom with router and round over bit.
Grain direction of bottom going from side to side makes rounding leading edge much less of an adventure.  Leave glue off first 2 to 3 inches at panel ends, use dry wood both to help with shrinkage.

daveg, SW Washington & AZ