Ilford makes a 6" filter set. You could cover a 9x12cm neg with that, or you could get a 6" or smaller reflector for your lamp and make a gel filter to fit over it. An advantage of using Ilford MG filters, is that they have neutral density built in, so you can go from grades 00-3.5 without having to adjust exposure, and grades 4-5 only need one extra stop.