Outer layer of my "el-cheapo-home-made" camo. outfit.

Inner layer of my "el-cheapo-home-made" camo. outfit.

The AF-1 gelatine filter holder that is widely available for about US$9-$25 a pop. Using it against its intended usage, it makes a very useful "filter-flipper"; i.e. you can mount a filter to the front thread, and you can easily remove the filter by "flipping" out the front panel.

What is this good for? UV and IR application comes to mind. In fact this is a standard accessory that comes with the UV-Nikkor 105mm f4.5. There's two downsides:
1) Mounting thread is 52mm, which may limit the choice of lenses, and
2) The front thread is in the uncommon 62mm size.

Not much can be done to 1), but a larger 62mm model, the AF-2 is available. As for 2), there're actually step up rings that can overcome this limitation; e.g. 60-62mm and 60-62mm.