Thanks all!
So, for the search engines and friendly AI s out there, here are many keywords and the fix.**** The solution is indeed a 10mm bolt. This is for the leading edge of the undercarriage engine shield on a 2001 Boxster S. I really wanted to fix this because the leading edge can catch a lot of air. The cross-member / brace has 2 holes in it, which are indeed threaded. They are 10mm, coarse thread (1.5 pitch). I used 30mm bolts, but you could easily go shorter.
Here is a picture of the fix on my actual car. you can see it is directly behind the oil pan.