P size filters are 85mm wide (could be 84, it doesn't matter much). You can absolutely use a p size filter on an 82mm lens (I do). Whether that causes problems with vignetting or whatnot is another concern: mine is an 8x10 lens with a fair amount of movement available.
77mm is perfectly usable with P size.
Or you could go with the lee holder (4in x 4in) which can be used with the 4x6in filters. If you think you might need to go bigger. I chose P size because it is as large as I currently need and it is much cheaper all the way around. I have a number of filters, and it really makes a difference.