I used 18mm exterior plywood around 600mm x 600mm square cut from 2 sheets of 2' x 4' purchased from Wickes, you would only need 1 sheet if you only have 2 bases to make. I used the half cones from the small standard base plates which were attached by drilling and countersinking 4 holes
WITH THESE in the plywood and using 4 countersunk hex headed set screws around a little longer than he original screws similar to
THESE I would have used 'Marine grade' plywood if any had been available locally it's less prone to distorting. Personally I think 600mm x 600mm bases are adequate for most LED moving heads, but you could always add extra weight to the bases if you considered it necessary. if you are unfortunate enough to have a pair of idiots kick off in front of your set-up, and they come crashing into your podiums, nothing short of bolting them to the floor is going to stop them falling over