To the best of my knowledge, you need 4 160mm (6.3 inches) bolts to retain your Mace 25mm insulator and 12mm plenum spacer. I'm talking about the 4 bolts that pass thru the plenum cover and IM.
I haven't installed my plenum spacer yet, but thats what I'm expecting to use.
They are 8mm X 1.25 X 160mm bolts.
I believe I have 1 extra- if you need it.