I came up with a quick and easy way to add a bit of cargo space using just the canvas shopping bags I already use at the grocery store:
After loading the pet carrier I connected one handle from each of the two bags with a little karabiner and put them them over the bike on either side: (Photos are below in reverse order)
Then dropped the seat to keep them in place and ran a bungee through the two remaining handles to take some of the wight and keep the bags from sagging outward:
The finished product rode pretty good at about 40 mph the 2 miles home. I don't know that I want to use this for over the road trips, but getting home from the store was fine.
I might have have one more layer of fabric added to the handles for a bit more strength but I won't make it a priority unless I see them showing any wear.