Karl,
On one of our large tractors, that gets lots of heavy work, we simply put the overflow tubing into a tall plastic
soda bottle.
Did not change the original radiator cap and simply sealed the
tubing in the bottle mouth with a bit of breathable foam to keep
out the dust. The original cap lets the water flow back into the
radiator as the whole system cools down. Works great, very simple.
Was planning to do the same on my TR2, but as long as I keep the
coolant level just a tad over the bottom of the filler chamber,
I have had no problems with overflows.