Simulation of a Rondo-inspired design for Betsy K
Based on an 'open bottom' enclosure from the Rondo, I made a quick design using the Betsy K driver and sim'd it in AkAbak.
The baffle is 24 in wide and make the curved rear sections such that the CSA is 384 square inches x 60 in tall, with 4 in legs to allow a gap of 4 in clearance at the floor. The lower window brace is assumed to be 0.5 in thick plywood and provides 80% of the max CSA or 307 square inches. The driver is mounted at 25 in from the top, and the speaker is placed at 70 inches away from the back wall (measured to the front baffle). It requires quite a bit of BSC to get the rising response to go down level: 6 ohm and 1.5 mH in parallel before positive connection to driver.
Simulation with no stuffing or damping and 2.83 V of driver input gives the following frequency response and cone displacement plots: