This little DIY "hack" from MrCheesyCam on YouTube shows you how to add a 3 stop ND filter to lenses that you might otherwise not be able to (such as the MFT Rokinon Fisheye as shown in the video). All the components needed for this nifty little mod cost well under $10.
Using the Rosco gel (#3404), you can cut down on the light coming hitting the sensor by 3 stops, allowing you to shoot wide open on your lens without having to jack up shutter speed or drop down into the "low" ISO settings.
Here's a comparison (from the video) of the effects of this mod:
Disclaimer: I haven't personally played around with this mod, I anticipate giving it a try with one of my fisheye lenses but there's always an element of risk when messing around with your camera or optics. Be careful!
If you do give the mod a shot be sure to tell us how it worked for you in the comments below.