was wondering if these exist?

I was sat wondering the other day, trying to decide between my hi-matic 7s or my zorki 4 and was disappointed not to choose the zorki, which I prefer, my reasoning was thus - in a pinch if I need to meter the minolta has it and I wont have to reach into my bag for the hand-held meter.

and as wondering tends to do, I was led to a place where I figured there must be a simple fix for this problem, to take away the need to choose, a little meter I could fit to my zorki hot shoe and never need to consider this again.

does such a wonderful thing exist?
cds or battery powered?