I will not be held responsible for damage you do to your equipment. This is only a recommendation. Please read in its entirety before modifying your system. If you are uncomfortable modifying your system to this level, don't do it. It should go without saying that it is dangerous to use power tools near sensitive computer equipment, so remove the internals from the case before attempting to modify it.
Using a dremel tool, power drill, or similar, cut an additional hole in your case for an intake fan(s) near the bottom of the case. Be sure to place this hole in a place where cold air from the room will be easily accessible to the fan. Ideally your intake should be as low possible, and as far from the video cards as possible. Best practice is to place the hole so that the hottest parts of the video cards lies near the center of a straight line from the center of the intake to the center of the exhaust. Drilling out multiple small holes works as well or better than cutting out an entire plate section, so long as you try to get a tight grouping of holes and end up with >50% empty space in the port area. Uncovering area larger than the fan blades is fundamentally counterproductive.
Securely attach a filter to the interior of the intake fan to prevent dust from entering the case.
Using your dremel tool, open up enough area where you will mount your exhaust fans to maximize their air flow. Any part of the case that is solid and obscures a fan blade is an immediate candidate for removal, but use your best judgement. When in doubt, do not remove anything, you can't put it back.
Now any parts of the intake and exhaust ports that do not block a fan blade should be covered, to maximize speed of air moving through the fan and minimize the potential for hot air to re-enter the case. Depending on the material of your case, and the tools available to you, there can be a number of options here. A good bet is to use something heat resistant that you can easily get an air-tight seal with. Simple solutions are heat-resistant plastic and super glue - pro solutions include highly conductive sheet metal welded to a metal frame. Use what you have on hand, ask if you're unsure whether its a good choice, YMMV.
Reassemble the computer and test it out. Once you have the measured average statistics of your rig over the course of a week, ask yourself why you didn't just build a custom water-cooling system and overclock all your GPUs.