I've been trying to make sure I get the right solar filter for the upcoming eclipse in the US in August, but I'm inexperienced so I wanted to ask if the filter I'm looking at is correct. The one I found is a Baader filter. I'm going to use a zoom lens on my Nikon D5300 (no telescope or spotting scope). The only issue I can see is the size: they only have 50 and 65 mm, and my lens is 55mm. Is this the correct filter? If so, how do I deal with the size?
That filter seems to have a weird mounting mechanism. The screws might scratch the lens exterior. Perhaps something like this Mr Seymour 55mm screw-on filter (Amazon link) would be a better option. I'm assuming your lens's filter diameter is 55mm.
I recommend that you just buy some Baader sheet material from the same vendor, and make your own filter holder. The reason being, Baader film is thin and somewhat fragile. If you perforate it or stretch it, you run the risk of the filter no longer performing its job (i.e., letting in too much light). You can make several films for your camera from a single roll, and have some ready to practice solar photography with, or apply to binoculars, etc.