Call it what you want, but it works. in the spare of the moment.
Go buy a 2 litre bottle of Coke or whatever your preference is & drink it.... clean out the bottle & peal of the label so no one knows :lol: & mount it w/ tye-wraps in the engine compartment & fill it a little over 1/2 way w/anti-freeze & there you have it...... a quick & cheap way to have a overfill bottle.
P.S. Keep the hose from the radiator cap a 1/2" off the bottom of the bottle & cut it @ an angle aprox 1" for a pick-up angle. Oh & I left off the cap. Hose dia. 5/16"
Just a thought, & I have it on my CJ & it works.
Have A Great Day!