Here are the bases if you have the larger boxes:
https://www.usaudiomart.com/details/650112163-acoustat-22-bases/
You can see they have a unique cut out, (actually they are a double layer towards the front for added support for the panels. 22 x 21 x 1.
If the interfaces are the smaller ones:
https://reverb.com/item/40237142-vintage-acoustat-model-2-two-electrostatic-panel-speakers-mk-121-b
These are just a flat base. 22 x 17 x 1.