You could have custom sized plinths from Symposium. These have been used a brand of Canadien speakers whose name I can't remember.
material for plinth for subwoofer IRS-V
I have a gracefully aged IRS-V set. The fiberglass/resin shell and epoxy-over plywood plinth is still in good shape, but somewhat outdated by what we now know about decoupling and spikes etc. So I am setting out to replace the plinths under the subwoofer towers with either 1 inch or thicker aluminum (24x24) or 4 inch thick solid maple board or even 1/2 inch thick steel. All of the above options will be matched with brass or steel pointed feet. Given the size (8ft) and weight (600lb) of these towers, and the very limited range it covers (sub 70hz only) the conventional wisdom on material (i.e. maple rules) does not seem to apply. Any suggestion?