Well, I wanted something more secure than velcro, so we went ahead and tried the under-the-arm-and-behind-the back method. It worked very well, with a buckle in the middle. That cape is going nowhere no matter the gust of wind that hits it!
But now, I want a replica Reeves' belt buckle. The half-round egg design. Im not good at making round things and the fabric buckle that came with the costume just isnt up to par. Anyone have ideas to make one or have a spare laying around?