For the light cuts you are talking about you could probably get away with an HSS end mill, although a Carbide one would be better. Good old paraffin is the best lubricant. Your problem will be speed, you really need to get it down to maybe 1000-1500 rpm. Or find someone who has a milling machine. If your slot goes right through then I would see if you can remove most of the material first using a jigsaw, so you are just using the bit for final sizing and to get a clean edge. The other issue is lubrication, you will need quite a bit to prevent spalling on the bit, not sure how you would do that on your router. You don't say how long the slot needs to be. Would it be practical to mount it on a pillar drill and just overlap the holes to get your slot, much easier to lubricate that way.