Nice work this weekend. Until you get a new drill, you could use square u-bolts to hold it down. I don't remember if it was suggested earlier or not, but you might want to weld some cross pieces on your firebox door on the inside to keep it from warping. It usually isn't a problem on the thicker steel doors but with the ~24 gauge steel on the tank you have, it might have some issues. It might not be a bad idea to put a piece just below your grate and about the same distance from the top on the door end as well. The vent end still has the integrity of rounded bottom, as well as, the heavy-duty insert you welded on.
Dont worry, Ill get the bolts in. There are other drills to destroy I just ran out of time. Your explanation for the firebox reinforcements make sense. I was just imagining something external and rounded that I had no idea how to fabricate. I don't want to lose any space inside the firebox so I will have to come up with something creative. Do I just need to reinforce the firebox? And just on the door end?