I use to own a custom metal pad. I could try to give you some advice, though I'm not sure if it'll help.
First off, make sure you either coat the metal with something to prevent rusting due to sweat, or ensure that the pad is cleaned after every use. It will become a problem if you let it go for too long.
The back bar should have supporting bars that come down to the floor. I'm not sure how stable your bar is. Mine was strong and had supporting back bars. Even with all of that, it still bent slightly after awhile.
This wasn't the pad I owned, but you can get the idea from here: (HUGE picture! sorry)
Lastly, sliding squares. This wasn't a problem for me at first, but after heavy use, the square tiles began to move around. I believe most of that was due to the holding screws loosening. The squares didn't move much, but it would cause buttons to die when they did, and in the middle of a game too. It was very annoying. I could expect having to stop what I'm doing, position the tiles back in place, and screw everything down at least once a session.
I hope this helps, and good luck! I've always wanted to do something like this. From the looks of it, you're already off to a good start.
Edit:
I found a picture of my old pad, but it cuts off right above the supporting bars. .__.