Just don't put one in. That's best on many fronts in my opinion.
Coolants leave huge messes on the floor, walls, and ceiling when used on classic manual machine tools. The coolant can easily develop a bacteria colony that leaves it nasty smelling. Water based coolants will rust most surfaces eventually. Water based coolants also have a terrible way of invading the apron and headstock on a lathe and rusting them from the inside out.
The easy way is to machine slower, reducing heat, you can use a larger corner radius in either milling cutter or single point tool to protect the edge tool. At least in the single point cutter it also requires more rigidity from your machine.
If you feel like your production rate is hampered and you are being economically held back by the ability to remove metal, then maybe its time for a machining or turning center which have their own enclosures and the rigidity to take full advantage of carbide edge tools. And the parts come out more repeatable as well and you do less physical work.
I have one part that I make where I slot 3/16" x ~7/8" deep for a customer. I've not ever had success using an XL end mill to make it and so I use a horizontal mill with a slotting cutter on an arbor, full depth single pass to completion. Significant heat is generated despite the lowest table feed rate I have, so I mix up some coolant (Rustlick 2000 synthetic water based IIRC) and use a pump spray bottle to apply it as needed. Try to clean and dry the machine at the end. One can also brush on oil or use drips as when tapping.