Comments about Canon Canon 48 Drop-in Filter Holder (Accepts 48mm Screw-in filters). Fits EF 200mm F/1.8L, EF 300mm F/2.8L, EF 400mm F/2.8L II, EF 500mm F/4.5L, EF 600mm F/4L:

A useful accessory for anyone with a Pre-Image Stabilized 300mm f2.8L to 600mm f4L Canon lens. The major problem nowadays is finding suitable 48mm filters, because the few that are available generally seem to have rings that are to deep to allow the holder/filter combination to be inserted into the lens. If you have alloy framed filters (e.g.B+W or Kenko), you can achieve a decent fit after spending 30 minutes filing down the ring with some fine-grade emory paper. Works perfectly. Kenko seems to be the brand with the only decent selection if you're looking for warming/cooling filters.