Reply to #8 I made mine about 18 years ago and memory is not so good at my age but here goes, I cut them out of what looks like 2.5 or 3.0mm mild steel sheet, A 90° bend and a small 90° return (I had access to a press in those days) to fit in the channel, For the slots I drilled a couple of holes close to each other and elongated them into the slots with (a lot) of filing. Tack welded 2 x M8 nuts then had them powder coated locally.