Many years ago I made my own radius blocks ( 4" and 8") by planing pieces of oak flat, then roughing in the radius on a router table(rout the deepest channel down the middle and each channel next to it gets more shallow). Then I would sand the radius into the roughed-in blocks by moving back and forth over the radius block sanding jig I made , which was a piece of lexan held down over two rods with wing nuts. The more you tighten down the wing nuts, the sharper the radius. I would adjust for the proper radius by laying a radius guage that I made (1/8" plexi glass that has the radius cut by using a dremel with router base and the edge guide, with the edge guide's metal rods replaced with much longer rods to make the dremel like a big compass). I would adjust the wing nuts until I could no longer get a .002 feeler gauge to fit between the radius gauge and lexan curved top of the jig. Sometimes I had a .001-.002 gap here and there. Then I would put coarse sandpaper on the lexan and move the radius block back and forth to sand the radius into it. I'd switch to finer grits. I would have guides to make sure the block stayed straight while I moved it back and forth. Then I would lay the finished block on a piece of glass that has a .005 flatness tolerance to make sure the radius block didn't rock on that. And also would check the lengthwise flatness of the block with a precision 24" straight edge to make sure the flatness wasn't off by anymore than .003 or about that. I made a whole set. from 7.25" all the way to 20" . Glued handles on all of them and put a couple "coats" of danish oil on them.
I don't know if they were any better than the Stew-mac ones, but I used very old, stable wood. I would imagine the Stew-Mac ones would probably warp a little with age, which would really suck, although they'd love for you to buy a new one then, I guess.
I don't use my blocks much anymore. I found out that a long, narrow and flat block is much better, because it's better to sand in the path of the strings, which creates the best compound radius. Changing radius blocks as you go up the neck is also an inferior way of doing it.
Using radius blocks is like they often do it in the factory (even machines that do it). Using a long flat block with the right technique allows you to surpass a factory fret-job. Why duplicate a factory fret-job when you can surpass it ?
I've said all this before on other boards and a lot of guys don't like to hear it.
I even made a 13" block that is 10" at one end and goes to 16" at the other end. I used it and will probably never use it again. It makes the fret-board weird somehow. It's just not the proper way to do it, in my opinion. I think you end up sanding more off the board than you want to.
I think the reason why my fret-jobs were still turning out very good when using the 12" and 16" radius blocks is because I would then level the frets with a flat block, and I would go in the path of the strings.
If you don't understand what i mean about "in the path of the strings", think about how the outer E strings do NOT run parallel with the 2 center strings.
Hard to explain, so I'll shut up now.