The backup GPS antenna for my GRT is mounted on a shelf under the cowling. I attached my shelf to the firewall but I've seen others attached to the baffles or to the engine mount with Adel clamps. Fiberglass and non-metallic paint won't block the signal. My #1 GPS antenna (for a 430W) is conventionally mounted on the fuselage behind the canopy.
If your -7 is a slider you've got a lot of metal in the roll bar, brace, compass, etc. blocking it on the glareshield assuming inside the windshield. Even worse if you recess it. Sealing is an issue if it's outside the windshield.