I think the ring kit is the only triangulation solution (of the three widely in use) that suffers from that concentricity problem… Both linkage kits use one of the bars to mark/drill the sled. I’m actually not sure how the ring is aligned but it always seemed a little sketch to me.
I would echo @dlang’s hole drilling suggestions above and add one more to the bottom of the list. When trying to drill a hole perpendicular to some material alone and without a drill press I have had great success using a small mirror or even better a CD, place the reflective side up with the hole in the CD where you want to drill, now you can see the reflection of your drill bit in the CD. (you drill through the hole in the CD) Sight down the bit and make the bit and the reflection line up. If they make a straight line then your hole is perpendicular to the CD/surface. I’ve done this a handful of times in a pinch.
In my opinion the bottom line is: a drill press is worth having. I bought mine for $50, it had been laying in the snow and rain on it’s side for 8 years. But it was built in the 50’s so it’s solid and amazing! After restoration I use it almost every single day - no joke.