When i'm in a bind and need an ND filter to fit over my wide angle lenses, here's a little tip on how I make a custom Wide Angle filter adapter. Most people will just tape an ND directly to a lens, but this method allows you to put one on and take it off just as quick. It a simple process that works with basic card stock formed into a cylinder and cut to size. [The firmer the card stock the better..]
Now what you want to make sure you do before starting this project is test the filter that you plan on using. These wide angle lenses must use oversized filters, and for my Panasonic 7-14mm i'm using an 86mm Variable ND. If you have to go bigger than this, Fotodiox makes ND Filter sizes up 145mm (found here).
Normally we've gone as far as 3D Printed adapters, but when you're pressed for time this works all the same. This trick works on just about any wide angle or fisheye lens so long as you have an oversized filter. You can use this method to hold circular or square filteres. It will also work on many Cinema Prime lenses that don't have front threads.
If you really want to get creative, you can make one of these attached to a step up filter ring (instead of directly to a filter). Now that you have a filter ring, you can swap the front filter with different types (polarizers, different density static NDs, gradients, etc).
Sorry again for the weird audio, shot this with an iphone in hand as one of those spontaneous things. Hope the tip helps, leave some comments.