1) Keep looking, you'll be able to find the expanded metal at a shop, reasonably priced and cut. You don't need to weld it, you can do all the work with washers, nuts and bolts.
2) I bought a 1" metal hole saw (bit) for $7 bucks at Menards. It worked fine.
3) I bought a 22 1/2" donor grill for $10 bucks and used the lid for the top of my UDS, the grill grate for one of my two cooking grates and the charcoal grate for the bottom of my charcoal basket.
4) For years I used a 26 1/2 inch Weber as a smoker and used a water pan to ad moisture. You had to add moisture because of the characteristics of the Weber. I think you'll find that the characteristics of the UDS are different, it's a very moist cooker.
If you still think you need to add moisture, I'll be glad to sell you the 17" terra cotta pot base I bought, I don't use it anymore.
Build it, use it and have fun.