I have a very simple and cheap DIY sight glass. I used 3/8 OD polycarbonate tubing. It is unbreakable and as clear as glass. I don't have a pic of it now, and I'm out of town for the week. Basically, I installed an angle compression fitting in lower part of the brew kettle (1/4" MPT to 3/8 compression). The PC tube is installed in the compression fitting and supported at the top (through the rim) by an eyebolt. The eye bolt is bolted to the rim and the ID of the eyebolt 3/8, so the PC tube fits through.
It works well, and PC is nearly unbreakable, so no protection is required.