I have an REI 27x27x27 roll-up table, but I've found that to be insufficient for cooking, washing dishes, etc. Previously we'd camped with a 6' aluminum table, carried either in the bed of the truck or on the roof rack. No longer having those options, I picked up a $35 plastic folding table from Walmart.
I was unsure of where I would put it initially, so I began experimenting. The first thing I tried was with it in the back under everything else. This actually worked great, except for one thing: when we pulled into camp, it was the first thing I wanted to setup.
I carried it vertically behind the front seats on the way home, but that prevented a sleepy passenger from being able to recline.
Next, I began experimenting with the OEM roof rack crossbars I was no longer using. I came up this, and have been very happy with it.
It's immediately accessible, and provides a flat platform to carry a chainsaw or similarly-sized item. I drilled holes through the table that lined up with the dimensions off the crossbars, and it's held in place with Grip Ties.
I'd like to add a fairing to help keep air from even flowing into that dead space, but I haven't done so yet, and I cannot be sure it would even make any noticeable difference in fuel economy.
Using and reconfiguring the load out is all part of the fun though!