Return the slot cooler you bought and buy one that pulls air in and blows over the video card. For example,
this one here.. The only problem with this idea though is that the cooler is going to be pulling in air right next to where the video card is expelling its hot air. After a while operating I'd imagine it starts to normalize out.
Also, are those load temps? If they are then you might be okay.
Also, consider possibly doing a mod to your side panel adding an intake fan. If you're not a modder, see if a side panel with a fan is made for your case.
And lastly... this is a "Duh" suggestion, but get a can of duster air and blow out any dust that may be collected in and on the card, as well as your case and case fans. Dust can block good airflows and cause problems, and it's essential to clean the case out everynow and then to keep the bunnies at bay.
These are just a couple of options you can investigate without voiding your warranty (on the video card)