Well if you feel ambitious but a plank of very dense hardwood. You can find two inch thick purple heart quite easily. Drill and tap three holes in it one in the front and two in the back. Put inserts into the holes and build some ready rod spikes with nuts on the to so you can level the speakers from the top. Measure in the stands you have built and level them your spikes that come with the speakers or I use disc's that cam with my speaker stand place them two at the front on at the rear measure the speakers on the stands. Then check out the level on the speakers and from side to side the angle on each the toe in compared to each other etc. You have a system that cam be adjusted easily that will anchor the speakers to the floor everything will be wonderful.
Regards