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You can make your own (for testing purposes) with a bit of thin cardboard or plastic and masking tape...but it won't (probably) do much good on a Jeep Liberty. Vortex generators cause the air to spin and hug a sharper curve than smooth airflow can. However on a Liberty there is very little curve to hug, it simply chops off at the back. You might see half a MPG dif.
If you have ever seen air spinning off the end of an airplane wing you get the concept of tabs. Tabs are tiny airplane wing tips. By angling the tabs slightly, you create higher pressure on the forward side and lower on the opposite. Thus the air will spill over the edge and cause a spin. Of course one could buy tabs that have been tested to maximize the efficiency of the effect, but even home-made tabs set at about the right angle will have enough effect for testing purposes...without costing hundreds of $. If you read the material about mounting the tabs you will probably glean enough info to figure out how and where to mount them. You might even plagiarize the general shape, though the shape of most are designed for attractiveness, not for efficiency. A flat/square wing creates the best vortex, but a sculpted sharp fin shape sells better. They are also designed for safety. A sharp wing edge is best, but the manufactures don't want to be sued...go figure. Tabs are placed in opposite angles, one left, next one right. This, I think, helps the vortex to carry far behind the vehicle without colliding with it's neighbor. Placing tabs too close will probably still cause interference.
The AirTabs brand is a special V design that builds two tabs into one unit... so don't copy them. Just make simple wings.
This is something that someone here could/should test. What is the best home-made tab shapes and angles. It would have to be someone with a sloped back window and someone who has regular enough commutes to get accurate results. I say "someone" because I don't have a regular commute and I am stuck in a city with VERY slow traffic.
Flat back vehicles like trailers and RVs do benefit from one other feature of vortex generators. They cause a reduced wake which somehow chops up the knock around effects of side wind gusts and passing trucks. I haven't read the science of it, but my theory is that when an RV is knocked around on the road by a passing truck it is due to the wind's effect on the RVs following wake, not so much the air hitting the RV's side. When the wake is knocked to one side it causes the RV to be pulled in that direction...and we all know the power of the following wake. I think that air tabs either block the wind a bit, or break up it's pull on the vehicle.
Incidentally, the divots in a golf ball cause the same effect. The air passing over the back of the ball can hug the ball to a greater angle. I think you should try tabs before drilling divots in your car though.
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