Interesting concept idea here that is fine on paper, but in real life, it just ain't gonna cut it--or certainly not worth the time and risk of problems while on the road. Problems of which there are a few, as has been mentioned.
A $15 perfectly fine aluminum rear rack will be secure, easily carry credit card minimum stuff and you can use p clamps if your bike doesn't have eyelets. And you can put heavy grease, wax or loctite on the bolts and they won't loosen.
but hey, have fun experimenting if you want, just be aware that this idea doesn't get used for a reason.