One option is to stack ND filters to reach the desired optical density.THIS IS A RECIPE FOR DISASTER.
If you're exposing either your eyes or your camera to dangerous sunlight such that you need protection in the first place, photographic filters of any kind and in any combination will NOT
provide said protection.
Now, you certainly don't always need protection when the sun hits your eyes or the front element of your camera. However, it really doesn't take very much to overdo it, especially
when you're adding optics to the mix.
If the camera or your eyes are going to be pointed at the sun for more than a second more than once or twice, you need some sort of real protection. For your eyes, that means welding glass or eclipse glasses or the like. For your camera most especially if you yourself will be looking through the viewfinder
that means either comparable protection or covering the front element (with the lens cap, etc.
) when you're not actively making an exposure.
For the eclipse next weekend, I'll have two cameras going. The 5DIII will have the TS-E 24 (possibly with the 1.4x or 2x TC), and the 5DII will have the 400 f/2.8 (with the biggest TC left over). The 400 will have a Baader filter and will double as a viewing telescope. For the 24, I'll frame the shot early, almost certainly well before the sun enters the frame. I'll then hang a hat or something over the front of the camera. When things get interesting, I'll remove the hat, remotely trigger a rapid-fire bracket, and hang the hat back on. Lather, rinse, repeat when the light changes enough to make it worthwhile. The whole time I'll have eclipse sunglasses either on my nose or perched on my forehead, depending on whether I'm looking at the sun or not.
I most strongly encourage others to do likewise.