Not easy to find, but I came across these Thiele small parameters for the 416A:

Model Number: 416A
Xmax (inch): 0.6
Xmax (mm): 15
Re (ohms): 12.54
Fs (Hz) 21.4
Vas (cu. ft.) 36.8
Qts 0.272
Le 2.5
BI 18.01
Cone Dia 12.7
Cms 0.001124

Plugging in to this calculator Speaker Box Enclosure Designer / Calculator I get 10.02ft3 for volume. 6" x 2.57" length port for the 416A.

That would give me a box OD 28" wide, 21" deep and 36" high made from 3/4 MDF with 3/4" battens and leave room on top to get the 811 to ear level.