Slotting cast iron question

[festal]

Hello All.  I know there are very good machinists and metal workers here.  I'm a glutton for punishment and recently picked up cast iron table saw table to use as my router wing extension.  Table is big 20" x 27" and the holes i need to slot are on the sides.  I managed to drill them out already but 3 holes need to be slotted to accommodate my table saw attachment points.
Here i need to slot it to the marker outline.  So far i tried a drill bit and endmill.  It works but very very slow process.  There is a support rib on the other side of this hole which doesn't help   I also have dremel and carbide stone but haven't tried it.  I don't have a die grinder, do have palm router but not sure its a good idea. 
View attachment 1

in the second picture i drilled out another hole and just need to connect them. 
View attachment 2

What is the easiest, safest and fast way of accomplishing this?

Thank you

 

[weekendwarrior]

The Domino would have slotted holes perfect for this situation!  You already started so I wouldn't recommend it at this point.    ;D ;D

 

[festal]

The Domino would have slotted holes perfect for this situation!  You already started so I wouldn't recommend it at this point.    ;D ;D

Domino into cast iron?  holes were there from factory i just needed to adjust them a bit

 

[Packard]

A Dremmel type machine with grinding tips would make quick work of this.  Or use a rat tail file and do it manually.  (But the Dremmel is going to be faster and easier.)


Rat tail file:

 

[festal]

A Dremmel type machine with grinding tips would make quick work of this.  Or use a rat tail file and do it manually.  (But the Dremmel is going to be faster and easier.)


Rat tail file:

Thank you. Will try dremel. Rat file is in transit

Sent from my iPhone using Tapatalk

 

[Peter_C]

I often use a carbid burr for extending holes, but a rat tail in all reality will be just about as fast as there is no setup, just grab file and begin.

 

[festal]

I often use a carbid burr for extending holes, but a rat tail in all reality will be just about as fast as there is no setup, just grab file and begin.

Thank you will try it