Man, I just priced a back horizontal Length panel for my 220/G, and it was $450+ 72X30x12mm
I'm assuming that back panel for the 180/G is about $400.
I'll be better off getting a cheap 180/g on Craigslist and tearing it apart to use the glass. Then at least I could get the panel and use the rest to build some QT/HT's and sell them. 12mm will go quick.
Poor spice will never find a 42" panel. Just one of those 18mm low iron will be $1,000+. Might be better to buy another aquarium on Craigs. Some cheap large ones. I got my 220 for $300. It even came with a mag 18 & 55/G. So really I stole that tank for $100.