Filter won't work... This lens has no threads for a filter. It would be a huge, expensive filter anyway... approx. 110mm. (The EF 300mm f/2.8L uses drop-in filters that fit into a slot toward the rear of the lens.)
That lens actually has a built-in "protective" filter... of sorts. The front element is "plain glass" and can be replaced independently. This "plain" element supposedly serves optical purposes, but also doubles as "protection" that can be replaced at lower cost than a complex shape element or group of elements. (However, Canon no longer services this lens because parts supplies have been exhausted.... so best to protect the optics!)
The hood you need is the "ET-120", which was also used on the original EF 400mm f/4 "DO" IS USM lens. If you find a brand new Canon OEM ET-120 anywhere (which I doubt will be possible), it will cost around $400.
I'd suggest looking for used... set up a "watch" on KEH and a search on eBay.... contact the lens rental shops... look one Craigslist... etc. The best "deal" will be someone who doesn't know how scarce those things are!
There are also ET-120B (for EF 200mm f/2L IS USM lens) and ET-120 II W (for EF 300mm f/2.8L IS USM "II" lens).... But I don't know if those will fit the older lens. Plus the OEM Canon hoods for these newer lenses are even more expensive than the one for the ET-120.
You might look into the sizes of the replacement hoods for Sigma 300mm f/2.8 APO and Sigma 120-300mm f/2.8 "Sport" lenses. I don't have either to compare, but looking at the latter and it's hood, and both appear very similar in size and design. That Sigma hood is available to purchase for $120.
I just measured the outer diameter of the Canon lens - the rim where the hood slides on - and it's about 117 or 118mm. Maybe the folks at Lensrentals or one of the similar shops can tell you... they probably have all the lenses with their respective hoods and could test fit them... or at least measure the hood and lens to give you the dimensions. The Sigma hood is black, but if it fits you could paint it or fit it with a Lens Coat or whatever... or just use it as is. (Some of the Nikon or Sony "big lens" hoods might fit, too... but they appear to be pretty pricey, like the Canon OEM hoods.)
Blair Shaw Jr wrote:
....Those lens Hoods are ridiculously over priced and Canon should not sell them separately as they have done....
Actually Canon
includes the hood with this lens... as they do the matched lens hood with all their L-series lenses. (Also included with the EF 300/2.8L IS USM are a special lens cap that covers the entire hood, a hard case, a tripod mounting ring and a gel "skylight" filter in a drop-in filter holder).
For these lenses, Canon sells
replacement hoods, in case the provided one gets damaged or is lost. I agree those hoods seem very pricey. I think I read that they're carbon fiber, which is nearly unbreakable. But, even so, at roughly $400 to $500, they're certainly expensive!
OTOH, Canon
does sell the hoods separately for all of their non-L-series lenses, which is kinda stingy of them... And the prices ain't cheap, though most are nowhere near as expensive as the "super tele" hoods. Most fo the plastic Canon lens hoods in smaller sizes sell for $20 to $30. Newer ones with the much-improved latching mechanism, are a bit more pricey... typically $45 to $75, depending upon the size. Fortunately there are cheaper third party "clones" for most, though. Vello sells many of them for 50% to 66% the price of the Canon OEM. Too bad they don't make hoods for the super teles!