Nicely done- adding the wheels will make your seat much better than sitting on a upside down bucket! (That's how I swap brake pads.) Can I ask- what process are you using? I see orange flecks in photo 1- is that residual slag from flux core or SMAW?

I had a hot flash while reading your post: if you plan to use expanded metal on the seat, you could fold the edges down to 90* all the way around, and attach on the inside surface of the seat rectangle instead of the top surface, and have a very smooth seat with no sharp edges to grab your clothes. Unless you thought of that already!