The “random pieces of metal” are the ones that survived re-entry and are durable enough for friction. Amass sufficient parts and it can totally work to make a suit in this hypothetical scenario that serves his specific problems.
“Also makes The Flash look like a thief”
So what? On the scale of moral compass, him stealing leftover shuttle parts left for scrap so he can use it for better deeds, isn’t bad as other crimes a speedster could commit potentially.