hello,you can purchase ND2-ND32 (1-5 Stop) Variable ND Lens Filter. Check here: https://www.kentfaith.com/index.php?route=product/search&search=KF01.1061 

It is a continuous spin without stops. 

This one is high quality materials, and contains both a variable ND filter + a polarizing filter. Also - most variable Nd filters show and “x” pattern when they’re turned to the max, essentially making them useless at full power. This one shows no “x” pattern. I’m happy I bought it! Very high quality and sturdy. 

I've never used Hoya ND's, but I have GOBE, KERNEL, ZOMEI, and K&F ND filters. Out of all of them, I like the K&F the most because the colors look great out of the camera and they don't create a vignette around the edges like some of my other nd's which I hate. Because of the great out of camera colors, it gives you more to work with on post-production when you shoot on a flat profile. Also, I've used them during night photography on bulb mode to do light painting and the 4 - 6 stop ones work great for this. Overall, I think for the money the K&F filters are worth it. Awesome product! 

No, the lens cap can only be used with K&F Concept variable ND filters.