1) Stop using the Slim 5. NEVER EVER use it.

2) You need to close the sides and back, and force vent with an intake fan panel in the bottom 2-3 spaces (FP2/3 or QFP2 suggested). If you already have something drawing out the top, you don't need another panel to pull.

3) For this to work, you're going to have to make sure the front of the rack is fully sealed. That means custom rackshelves, tight fitting rack mount equipment, or the security plexi panels over gear you can't do that to. You want to ensure cold air comes in the bottom (and nowhere else) and chimneys out the top (and nowhere else) - somewhere else causes a "short circuit" and defeats your cooling.

PS Stop using the Slim5 immediately.