IIRC the cover has holes already. I've never had the voltage sag with use, I suppose it would, especially on a bright winter's day - but so far so good.

I'm pretty careful not to have the meter on unless I'm actually using it, also I only use the FTN meter for macro where TTL metering is nice to have. The rest of my metering is done with a LunaSix or a Weston.