In my travels I came across some interesting cheap filters on ebay. Sellers advertise them as IR infrared filters, but I noticed that one of their IR filters has 650nm on it. With Ha at 656nm I thought this might make it for a wide band Ha filter. At $15 including free freight from China it was cheap enough, although I don't know why I bought it since my DSLR isn't sensitive to Ha anyway lol.
Since I couldn't find any with a 48mm thread, I bought a 46mm filter and 46-48mm step ring. It perfectly fits into any 2" filter thread now.
I might mess about with it if it's clear later and see how it goes.