Note that despite the picture at Moss, the bar (at least one for MKI Midget) doesn't go straight across (it would hit the seat) it curves backward (toward rear of car, than across). You could make your own I'm sure but so much easier to find used if you can, $100 for a set at Moss is nuts (for my vintage anyway). A mix of proper gauge conduit and elbows and all might do it nicely for cheap.
I like to ride with tonneau on covering the passenger side. Looks cool, keeps your stuff in the car, easy to "secure" your vehicle when you get where you're going, protects the interior from sun damage too. I like tonneaus, can you tell? Maybe even reduce drag a bit and you go FASTER....maybe not.