Tim,
By all means, If I had the materials, tools, and the ability, I would make my own also.
I bought mine as a package with the rotisserie motor, spit rod, etc..
I just checked their web site and they sell just the Stainless Steel Ring by itself for $119.00 (OUCH), I don't remember it being that much back when I purchased my package from them, a year or so ago.
This ring is only a little over 5" (Maybe 5.125") tall overall.
From the top of the Kettle lip to the top of the roti ring, is just shy of 5" by an 8th" or so
The hole that the spit rod passes through, is only 2 & 7/8" (2.875")from the top of the kettle lip, to the hole on center.
I recently saw a post from the guy that makes that Rib-O-Lator, ferris wheel contraption, he said that if the spit rod location is at least 6" above cooking grate level, in a kettle, then you could use the Rib-O-Lator, with a standard grill grate in place.
So that could be something you may want to consider in your Roti-Ring design
If I were to design and build my own, as you intend to, I would make the ring tall enough to accomidate a Rib-O-Lator, just in case I decide I have to have one at some point in time!
I can see the advantage of having it tall enough that you can have your cooking grate in place, and not interfere with something big on the spit rod, say a roti basket for example. That way I could place things to cook on the sides of the cook grate, as well as having the roti going...
Here is a picture of the ring in standard kettle mode, kettle lid on
Kettle mode lid off
Kettle mode (cleaning configuration, WSM lid fits like on it securely this way also)
This is flipped over (WSM Mode)(fits onto a Kettle bowl either way, but a Kettle lid would not sit on it this way)
I hope these dimensions and photos help you decide what is right for your particular needs, keep us posted, I am sure others will be interested to see what you come up with.... Photos Required
Corey