I love simple, easy to implement solutions to a common problem. The problem in this case, is using any sort of ND, polarizer, or other lens filter on wide-angle lenses that don't have filter threads. Sure, there are filter holder solutions but those can be a bit pricey for a hobbyist. In this video from MrCheesyCam, we're shown a simple way to DIY a filter onto a lens with some tape and card stock.
In the example video, the Panasonic Lumix 7-14mm is used in conjunction with an 86mm ND filter, and it's important to note that you'll need the filter to be a bit bigger than the lens, so that you have a better chance at avoiding any lens vignetting. As someone who owns this particular lens, I've definitely wished for screw-on filters since I'll often shoot video outside, and I need ND to bring my shutter and aperture down to the settings I want.
There are other techniques out there, including using ND gels on the back of the lens, that might work depending on the model of lens you're working with. Funny enough, when searching for examples of this to link to, I found another video from MrCheesyCam:
What other exposure tricks do you use? I'm personally a fan of using my iPhone in place of a grad-ND filter when shooting for fun while traveling or on vacation, dragging the phone over the lens during a long exposure. I wrote an article about that trick here.