if you phone ML, they will know the sizing and perhaps even sell you some - i think that i was missing one bolt or nut from one of my used pairs and they sold them to me.
but if you are too itchy to wait, i'll try to remember (both my pairs my scripts are still boxed up, but if i remember correctly .... )
there is a bolt coming out of the bottom of the script and a nut hole on the top, and i believe them to be the same size. so you can measure the bolt diamter and get the appropriate size at HD. i don't remember them being metric sizes. ML uses a wingnut.
you will also need some large OD (small ID) rubber washers to go between the scripts and the brackets. they let you finger tighten the bolts and get enough friction that the speaker stays put.
the washers i got were 2" in diameter but only a bolt sized hole in the center. picked them up at HD.