I have just bought a used pair of these isolation platforms made by studiospares to try and cure the slight bass issues I have due to having suspended wooden floors, at the moment I’m just using two granite chopping boards.
I’ve read a couple of very good user reviews about these from owners who are very impressed with the effect they have had.
they are 570mm x 370mm x 68mm and can take up to 135 Kg each. There isn’t a great deal about their construction on the studiospares website but for the money they are worth a shot.
As soon as they are installed I’ll post some pictures and right a small review. They also look perfect for a smallish sub.