I'll be using the Spark in a bright mountain landscape, before the July 20 day that most aftermarket ND filters are looking to ship.

Any thoughts on where to get a small or customizable ND filter? Something light that I can tape onto the gimbal temporarily.

Simple.  Buy ND gel, cut slightly bigger than lens size, tape over the lens.  something like https://www.amazon.com/Gel-Sheet ... nsity/dp/B0089R864I

These would be good if they fit the Spark http://amzn.to/2tgsl9e

Natemare13 Posted at 2017-6-18 05:38
Simple.  Buy ND gel, cut slightly bigger than lens size, tape over the lens.  something like https://www.amazon.com/Gel-Sheet-299-Neutral-Density/dp/B0089R864I

Thanks for the idea! Do you happen to know if optical gel sheet is good enough for high quality video? Eg the Mavic ND filters are glass, leading to high image quality.

10000% yes.  Used in motion pictures and TV all the time.