Corrected for tire size with JScan and still running the stock 3.73s, I averaged 17mpg on a trip from Nebraska to San Diego and 14 mpg on the way back due to some brutal head and side winds.
I am currently averaging 14 mpg around town.
I have a Rough Country hard tri-fold cover with drain tubes. I used the holes shown by the OP to run the tubes through and did not trim or shorten the drain tubes. I took a couple of pics of where the tube comes out, hopefully it is somewhat helpful to those asking.
I bought this one not too long ago to keep our English Springer Spaniel in the back. Works well especially if we go through a drive-thru. The openings may be too big for your Shih Tzu though...
I'm a fan of a white JT with black wheels. In fact, I nearly bought a white one but found a black one setup the way I wanted.
Here's a pic of the same Fuel Rebel 5 wheels in black to give you an idea of what they look like in black.