Hmm. Home built... I don't really like a lot of home built pictures I've seen - the main problem probably being imprecision in the hole causing excessive softness. I thought this product might get around that. But then again, Denver Camera Exchange has a whole bin full of spare caps. It shouldn't be too hard to find somebody to laser drill a hole in a piece of soda can. Average, you might be on to something. Either that or your on something.
My original question was meant more along the lines of "I'm thinking that to use this I would replace the existing cap, put it on bulb setting with a cable release and use the mirror as the shutter." Does that sound right?