For years I kept adding a small amount of water and coolant to my 37 stock radiator every couple of months. The situation was exactly like yours.
I finally installed an overflow tank by attaching a rubber hose to the overflow line and running it into a small overflow container I bought at the local parts store. Now I've noticed that after a long drive it will have a couple of cups of coolant in it from the normal expansion of warm fluid, but after parked, as the engine cools down and the systems fluid contracts, it will pull the collant back into the system and leave about a tablespoon of collant in the overflow tank. This, of course, is how todays modern systems work, except this is very low pressure. The principle is still the same and I rarely have to add collant to the system. Be sure the hose goes into the overflow container to the bottom and be sure the end of the hose stays in the fluid at all times so as it pulls water out it doesn't pull air. This will necessitate some fluid stays in the container at all times.
