I had a character roll exceptional success in designing a 600 ton starship, how should I reward this?
The ship he designed isn't the one the PCs are using, correct? If so, the accomplishment occurred in his past. It's part of his backstory, it's not something which directly effects his day-to-day life.
You're looking at "rewards" like professional renown or contacts in certain circles. There could have a monetary reward/bonus from his old employers, but how much of that remains is another question.
The real question here isn't what the reward for that success in the past was, but how will you plausibly integrate that success into the character's backstory. The character used to work as a naval architect and was successful in the career, so why is he no longer doing it? Why are they now a Traveller? Why aren't they still working in their previous career?
All the various bells and whistles in
MgT chargen are of no real use if they produce results which are not
plausible for you and your players. It's akin to
CT's sysgen; sometimes the system produces results that cannot be plausibly explained.
Remember, this plausibility hurdle applies to
player characters. You have to explain why this particular character is plausibly part of this particular party. You don't need to explain NPCs in the same manner however.
Chargen produced all sorts of oddities for me over the years and I greedily kept some of the better ones as recurring NPCs. I've written about a "golden boy" ex-IN captain who earned a knighthood during his career. That NPC was rolled up with
LBB:5's chargen and, while I felt he was implausible as a PC in the kind of campaigns I was running, I had no hesitation using him as a recurring NPC in several campaigns.