For the personal statement: don’t tone it down just because it’s a technical fellowship! Make sure to try and grab their attention and show how you are unique as a candidate. For the proposal, make sure it’s as thorough as you can (for what makes sense at your current level). I made it clear what equipment I would even be using and let them know that it was available at my school. That way they can see the experiment is not only worth doing (base that off NASA Tabs) but also I have most of what I need to get it done.
I would also talk about a specific NASA center for the visiting technologist aspect too. Talk about what center has core competence in your area and why going there would make sense!