That's the tricky part, I looked and looked, I think a person could use a reservoir out of a magnuson kit for 5th gen Camaro. It is small and mounts to the firewall, I however figured that the more capacity I had for coolant the better it may cool.
So I looked up canton racing and ordered a blem tank( small weld splatter on the aluminum) It is a batter box style tank for a mustang. I built a frame much like a battery box out of 1 inch angle. I mounted it to the radiator support and to the inner fender on the passengers side.
I then went and bought several different length 90 degree bend hoses, say 4 inches, then 14 or 16 or 18. They run down between the tank and radiator and under the radiator support to the front of vehicle where the intercooler pump is mounted. then out of pump into the heat exchanger.
I ordered a pump from lingenfelter it had the harness and everything with it.
Good luck, Hope I helped you out,
https://www.cantonracingproducts.com...5-10-MUSTANGS/
look up gates 28474 hose. You can get a different ones with various lengths, then cut to fit your setup.