The plate is more like an I shape with two bits removed to create a straigh Z

It can be home made by just taking a strip of aluminium and bending the ends in opposite direction and to a 90 degree angle.

So each lip is riveted on different sides of the scribe line. One in front of it and one to the back of it.

I attached two single blocks to it. That was cheaper in my case, but a double block is just as good as far as I can determine. However this does mean you are either using a cascading downhaul of 12:1 (recommended) or only a simple conventional downhaul of 8:1 (not recommended)

An alternative to the z-plate is to use a single bolt that passes through the mast from side to side. This one is both simple and allows easy assembling/disassembling. Latter is especially attractive in case of mishaps or repairs. Some way to position the blocks to the bolt is adviced however. Advantages of Z plat are lightweight, cost and easy positioning of the blocks relative to eachother

I hope this helps.

Wouter