I put the floor of the lower shelf in today. Expanded metal. It was a treat figuring out how to weld it in without just melting it to nothingness. The two things I figured out were, 1) weld on the thick part where the two pieces come together, not the thin part where I made the cut, and 2) start the puddle on the angle iron, then just barely flick it up until you see it bond with the expanded metal. I put on a few other small touches, like a bar for clamp storage, a flat piece to put the ground clamp on, and I cut the "U" off of an old, rusted U-bolt to make a hanger for my stinger.
Next step is to weld on the top. It's a 1/2" piece of plate, 3'x2' in size. I only plan to put a few small stitches to hold it in place, in case I ever need to take it off for anything. My plan is to just lay it on the ground, then set the frame on top of it, then weld, then turn the whole thing back over. After that, I'm going to paint it, install the casters, and install a 120v receptacle for accessories, and I should be good to go!