Thank you all for the help, this is a lifesaver. Such a stupid reason to miss almost 2 weeks of riding... I kinda like the ebay one with the metal safety cap, though after this I will probably zip-tie it anyway just to be safe.
Back when I was a Harley Mechanic, I invested in a MightyVac. Brake fluid is cheap, easy to change with the correct equipment. I change mine at least twice a year. I can't abide a spongy brake http://mityvac.com/pages/products_hvpo.asp
Alternatively, get a reservoir with a straight outlet like this http://tinyurl.com/pzfhpmg Due to a slightly different mounting angle, the top cap sits wedged between the frame-rails when done up....and can't go anywhere. More pics and detail here http://www.cafehusky.com/threads/pottering-around.39580/page-6#post-438597