I keep battling with design issues. The core problem is that the adjustment screws for the diagonal have to be accessible from above, so the place where the spider holds the diagonal holder cannot be larger than about 5/16". So the existing spider will not do. I need a spider that has a holder a little smaller.
Apparent solution: Cut a .3" x .3' x 1" piece of aluminum, drill and tap a 1/4"-20 hole through it.
I tried to machine that size, but I could not machine such a small piece. The mill vise jaws are a bit too tall. Using parallels was too hard. Enough height to clear the jaws meant not enough grip to hold the workpiece. Ordinarily I use parallel flats UNDER the workpiece to prop the workpiece up, but that requires props thinner than the workpiece. Even with all the scrap around the shop, I could find nothing, so I am going to cut them out of pieces of bar stock that I buy.
But I need to replace the chop saw blade first. It is a year and a half old, and doing a terrible job making smooth cuts. Of course the nut that holds the blade is a little rounded from using monkey wrenches, so I need to buy a 1/2" end wrench today. Yes, I am amazed that I do not have one. Then a lefthand nut. No, the nut has a hex key inside that will not turn! This is a terrible design; hold down a locking button while turning the hex key, so you do not have a free hand to use a rubber mallet to knock the wrench hard enough to break whatever corrosion is holding in place. I guess I will go to Home Depot tool repair.
Swayze Optical (a widely recognized mirror maker) has a very clever way to make a spider at http://www.swayzeoptical.com/spider/spider.html. Instead of his suggestion, I am using hacksaw blades. They are thin, springy, and cheap.
Why 1/4*20? If it is used as an adjuster, I would consider 10*32 for adjustability. You can find lengths of allthread in the metal area of a hardware store, or bracing for screen doors. Those might have mounting tabs at one end. Left and right hand thread versions, IIRC.
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