Caribou, have you ever considered getting a scraper? it's a simple razor blade (straight blade) sold at almost every store in the paint section, it's used to remove paint from glass, it will remove that calcium with 1 pass and no soaking in vinegar.. Just a thought.
On topic, I recently build a canopy for my 75, made it from 1x2 pine (like 50cents for 8 feet), then covered the frame with hobby panels from home depot, then used oak trim to finish the outside. built the frame with openings at the back for filters, it currently houses 2 2x4' light fixtures and my old single aquarium fixture (200w). all about 1' above the water, if a fish jumps it will at worst land on a ledge and can jump back in (flapping around) as there is no where for it to get out. The front slides up on "rails" (corner molding attached on 1 side only)..
120 Gal SW, 150lbs Prem. Fiji, 23Gal Sump, 2x250w 15K MH, 4x65w Actinic
3 x Halichoeres Chrysus (1 surfed), 3 x Nemateleotris magnifica, 1 x Centropyge bicolor, 11 x Scarlet Hermits, 6 x Zebra Hermits, 40 x Astrea Snails, 6 x Nassarius Snails, 3 x Cerith Snails