Working with aluminum tube you need to think about "annealing". I think that's the right word?
If you ever tried to flatten an end of a tube and it split. It's because it was heated then cooled quick to make it stiff. Something you would want in a thwart but not in a tube for a slider seat. For that you'll want to heat the tubes and let them cool slowly. That makes it slightly springy and less likely to crack over time.

The thwart tubes can be stiffened again after flattening the ends by heating again and cooling fast in water.