Odille,
As you know, a Cokin P holds the filter off the lens with gaps top & bottom. Because a solar filter is so dark, I suspect you'll see stray light entering from the top & bottom if you mount it in in a type P holder. Try it if you've already got it. A black polymer filter might have a better chance of working though using a P holder I suspect you might see something resembling flare or fog. A piece of Baader astrosolar or TO RG film or any other reflective metal coating type will almost certainly show reflections in the P mount.
Solar filters are usually mounted to the optic with a light tight mounting. It must be secure so it doesn't blow off. For a solar eclipse, the secure mounting must be able to be removed quickly and easily during the 15 -30 seconds just before totality without shifting the focus on a camera lens or shifting the pointing of the scope or lens.
There's an article on my web page that describes an easy way to make a light tight easy release mount from thin film.
http://joe-cali.com/eclipses/EQUIPME...arfilters.html
cheers
Joe Cali
http://joe-cali.com/eclipses