there are a bunch of ways to handle this. The best is this Richard Sachs seat pinch bolt.
RS Seat Lug Bolt + Nut Assembly - Richard Sachs Cycles
I have one (on my RS) and it is ultimate.
HOWEVER here's what I would do in your situation. take the existing "nut" that you have and find an M5 metric bolt that is the right length
I think the problem is that the "cro mo" bolt (the part that stripped) had to be machined down and then turned for threads.
and just because it SAYS Cro-mo on it does not mean that it is. There was a helicopter part scandal years ago that had to do with mismarked / counterfeit bolts.
Get a high quality M5 bolt that is the right length and use that
/markp