Canon 300 f 2.8 II Filter

Question - can you use a regular filter (52mm) in the filter holder or do you need to buy the Canon PL-C 52WII 52mm Drop-In Circular Polarizing Filter and use that?

Thanks.

As far as I know you have to buy the Canon one. My lens is the Canon 300 F2.8 L IS Mk1 = same filters. They are expensive and, in my experience, not used often.
There was (is?) a Canon filter holder that would accept 52mm filters but they had to be very slim and, if using a CPL, you couldn't rotate it without removing it.
Having said that there are few conditions where I would want to use a CPL on my 300 or 800 mm lenses - but there is the odd occasion when they can be useful - but would they be useful to you?

After owning Canon's drop in filter for over 7 years, I have only used it several times. If you shoot around a lot of water or reflective surfaces you might get more use from one.