Well after a few of those Shop pictures I saw over yonder in that Show Me Your Shop thread early this morning...

I went and cleaned mine fer awhile!
Then I encountered that Flat Rate Box Dilemma. Not to the extent of some guys who shall remain nameless,

but yeah I got a few sitting around that I don't have shelf space for. So I was trying to sort out where I was going to put them while putting other goodies away, and I kept running over this pile of leftover 3/4" plywood flooring that has been in my way since the house got remodeled a couple years ago, and I was seriously thinking on building shelf space, but really couldn't figure out where I want to put it. Presently I'm doing my wood work in the middle shop bay, but I want to move the woodworking into the south bay, close it up, insulate, and air condition it. That will also allow me to lock it, while the remainder of the shop is accessible.
Went to put some spare tools in my little mobile work bench I built years ago, and low and behold there lays a set of 4 casters I bought ages ago to build another mobile work bench, and the little light bulb clicked on over my head and...
I built a nice little mobile Flat Rate Box Storage Shelf with work space on top. Little tall for counter top, but it'll be sweet for assembling pens! Or whatever; no bending over to do it at standard counter height!
41" to bench top - top is 48 3/4" x 22 1/2" with trim.
Cabinet is 40" wide x 21" deep - Shelves are 12" high inside the frame. About 10 inches clearance getting in there.
1 - MFRB for reference to available space, there's room for a row of them sideways behind that that.