Step 7: Bottoms up...
Since there is no structural integrity required, this is just a "L" shaped piece of cardboard that is fitted to the width of the gap and taped on to the side consoles.
I just covered this with white duct tape that I had. I had already cleaned up the paintbrush and the duct tape matched the light color of the primer.
As you can see, it is pieced together with leftover cardboard. It would be nice to have a single flat piece of cardboard to use.
I did not make the round skirt for the chair bottom. It would interfere with the comfort adjustments of the chair too much. If you wanted to make it, it would just be a cylindrical wraparound which could be a tube within a tube to make it height adjustable and with some cutouts for any lever controls on the chair. Then again, you can bolt it to a pedestal just like the real chair if you wanted.