One of the dangers facing your beer as it ferments is the possibility of being “light struck”. This happens when the energy from light rays interacts with hop compounds in your beer to produce off flavors. If you use an opaque fermenter, this is not a danger to you. However, if you use clear glass or plastic containers, you need to protect your brew to keep it tasty.
You can purchase a Carboy Cover to protect your fermenting beer. But, personally, I don’t see a reason to spend $15-$30 for a commercially-produced gadget you can easily create from something you probably have crumpled in the bottom of your linen closet.
All you need to do is find one of your old pillowcases that have slowly worked their way to the bottom of the pile of useful items in your linen closet.
Take your pillowcase and slip it over the top of your carboy to make sure it is large enough to cover it without getting stuck. Lay the pillowcase on a flat surface so you can find the middle of the sewed end of the case.
Since you know the pillowcase fits over the top of your carboy, now you need to make sure the hole is large enough to accommodate the neck of your fermenter. Slip the case completely over the carboy to determine if you need to re-size the hole. It is better to start with a hole that is too small and enlarge it than cutting the hole too large, causing the cover to expose the top of your carboy to light.
The next time you are fermenting a batch of beer, now you can slip your new cover over your batch of beer to protect it from light. I usually put the cover on after I have attached the airlock, but if you are using a blow-off tube you will want to place the cover on first.