I assume you are talking about using an inner cover with a telescoping cover. If you use a migratory lid, no inner cover is needed, and no, they won't get too hot. If you are worried about too much sun, shades would be the way to go.
I know one guy who uses the inner cover with the hole plugged for his summer cover. It collects a bit of water, but doesn't seem to hurt. I know others that just use a 16 x 20 piece of cheap plywood for a cover. Works, but looks like crap.
If you already have telescoping covers, and use them without inner covers they can be a real btch to get off once they are glued down. If you don't want to spend the extra money on inner covers (they are nice), here's a couple of alternatives:
Heavy plastic sheet just a little bigger than hive top (20 x 24, say). Or, -- burlap sacks make real good inner covers, and they insulate too. A piece of 3/8 plywood 16 x 20 will work and is cheap enough. No need for a hole, unless for a bucket feeder.