I use a Rola rack that's mounted to the existing rails on my Subaru Outback, leave it on all the time as it's a bit of a pain to take on and off and even more of a pain to store it.

It affects my gas mileage a bit but I can live with that.

Very useful as it allows me to shift stuff from the back of the car to the roof when I have passengers, I've also got some very bright back up lights mounted onto it. I've made myself a dual 5 gallon gas can carrier and a full sized spare tire carrier that attach to it that are used when needed as well.

I keep a bag of bungie straps and ratchet straps in the car all the time for attaching things to the rack.

I've thought about getting one of the hitch receiver type of trays but I don't really want to have a hitch on the car. The Subaru's main issue with offroading is it's approach angles, a hitch would make that even worse plus I have to have access to the cargo area in the back all the time and the hitch type of tray would interfere with that.