There really is a large difference in accuracy between hydrometers and refractometers. I wouldn't ever trust a hydrometer for a reef tank with SPS corals.
A plexiglass top really isn't ideal on a saltwater tank. Speaking from experience, the salt creep over time is very hard to clean off and it will start to impede the light. As Flagg mentioned as well, it also doesn't allow good gas exchange in the system. Also, while it will lower your evaporation, evaporation is actually really effective for cooling the tank. In the summer, your tank could easily overheat without evaporative cooling. I assume you don't have a chiller? For a lid, I'd definitely recommend a screen lid. You can make one yourself by buying screen frame from a hardware store that you can cut to the right length. You'll also need the corner pieces and spline, which are also sold at the hardware store. The regular screen sold at the hardware store will block too much light, so people use clear netting, which you can buy from Bulk Reef Supply or potentially a craft store or local fish store. Bulk Reef Supply and Marine Depot also sell complete DIY screen lid kits.
