Beginners rigging question...
The "other stay" you speak of is called a topping lift. This is a line that attaches to the end of the boom, goes to the top of the mast and passes through a small block then down to a cleat located in the vacinity of the "gooseneck" (the boom attachment point). Since you mention an eyebolt atop the mast and the extra cleat portside the boom, I would bet this is where the topping lift should be rigged. When sailing the topping lift is slacked to allow proper mainsail trim then tightened before you drop the sail to keep the boom out of the cockpit. The line you can use for your topping lift can be relatively light as it bears very little weight. I would suggest you get some inexpensive 1/4" "XL" Sampson yachtbraid from Defender Industires for this purpose.
Good luck and smooth sailing.