A few observations...STBs are more helpful if they are fixed with no pivot points. I can certainly understand the reasons why people wouldn't want a rollbar, however each Miata I have owned has one for chassis rigidity. I also added bolt-in doorbars, which were the greatest addition to reduce NVH and additional rigidity. This was especially true in the NA.

Seam welding is the most laborious method, but least intrusive. It still won't add as much rigidity as a triangulated system. I'm not an engineer, though.