Well, back when I started, this is the chart I used, so ~6.25-12 ft^3 for 40 Hz Fs: http://www.lansingheritage.org/images/altec/plans/1960's-lf-design/page03.jpg
This later chart goes from 2.5-12 ft^3 for 39 Hz Fs: http://www.lansingheritage.org/images/altec/plans/1968-enclosure/page04.jpg
FWIW, in doing a bit of specs scaling using the 420's, 414's specs, I get a near 10 ft^3 cab, so within the old chart's limits.
Assuming a ~38" ear height, 28.5" W, 3/4" thick panels:
58.5" H
10.375" D
driver down 21.25" from top
8" dia. vent hole down 50.125" for ~39 Hz tuning, so either damp it or drop to a 6" dia. or add some length if a lower tuning is desired
All dims inside [i.d.]
Damp the top, one side, back ~2/3rds the way down with whatever you prefer or loosely fill with polyfil just enough to get rid of any 'hollowness'.
GM
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