Here is the reason, with the box material being so thin (1/16), the flat head cap screw won't tighten down properly. The screw would be snug, but the box would be loose. 
 

Like I said earlier, I made the sides the same length as the sub plate. This means there is no overlap, but that's what I wanted. The reason is so I'll have a built in 'weld prep' like I did with the safe box (the thicker material I welded earlier). I'll weld on this one soon, but right now I have more holes to put in this box.
 

The next step for my four piece box is to put in some clearance holes for the lock bolts. In order to aid me in doing that I made a small transfer punch. It's made out of mild steel and it only took me a few minutes to make, but it's not heat treated. No problem because I don't need much of a dent, just a scribed dot will do.
 

Here are what transfer punches look like. I know I've been talking about them and I thought it was time I show you what they look like. I've had this set for a very long time and come to find out they are not very good. The points are not heat treated, which is a bad thing because one or two hits on some steel and the tip flattens out. Looks like it's time for me to buy some good ones.
 

Anyways, with the limited amount of space I have to work with, the longer ones just won't fit. This is why I made the shorter one. I removed all the lock bolts and then placed the angle iron next to my new box, tapped it lightly and that's all it took. This would give me 'one' location but the other I would use my digital readout to find the other location.
 

If you look close you'll see a tiny dot that has been 'transferred' to my work piece. That's all I would need and then I used my center finder to pick up that spot. Now I could set my Zero on my digital readout and dial over to the other one.