I wanted a FF3 protector that covered the top of the sight vs being open. The Eotech MRDS protector fits the FF3, but needs a little "adjustment" to access the power button on the FF3. They are not available from a retailer, but you can order the MRDS protector directly from Eotech.
To access the power button I drilled a starter hole then used a file to create an oval opening that matched the button position on the FF3. A little sharpie to color the bare metal finished the job. I went as far as to sharpie the camo pattern on the protector.