I own a gun store and I get asked this question quite a bit. I have been running a product called RackBone for a while. I works pretty well. I think they have been recently bought out because their website is down right now, but you may be able to find a used one. Another (more expensive ) option is to go with a Blac-Rac retention system. I will be adding one to my 4Runner soon. I am going to put a cross member in using the mounting points for the back seat hand rails. This will keep it up tight against the roof and it will also work as a hand support since you will need to remove the OEM handles. One of the guys that owns Blac-Rac came by the store the other day and this was the system he was using. Looked great. Did not interfere with head room and you could not see if from the outside unless you were deliberately looking for it. I'll post pics when I get mine done.