This isn't the first time I've used this little trick, but since I needed to make a few more filters for some upcoming solar observations, I thought I'd share.
You know those disposable sunglasses you get after an eye exam? They are an excellent source for neutral density filters, if they are simple dark tinted, or lunar filters if they have a brown tint. Simply cut out 1 1/4" (a little less than 32mm) disks from the material. They have the potential to supply up to six disks, which is more than adequate for most amateur astronomers. They can always be layered to add more filtration as needed. The way you use them is to put them ahead of the eyepiece if you have a diagonal. If you are using a telescope that doesn't use diagonals, say a reflector, you may need to do a little engineering, perhaps making a hood that attaches over the back of the eyepiece, putting the filter between the eye and the optics.
They help to tone down the Moon for lunar observations, and can be used in conjunction with solar filters to provide additional contrast (NEVER use these filters alone for observing the Sun, even layered. Always use proper solar filtration).
So, next eye exam, don't toss these little disposable sunglasses, add them to your astronomical tool kit.