First up is Dave Groski's famous design, check out his website for a decent description:
And that filter pair could be bought from ebay, slightly more expensive than the above website mentioned:
And actually, this possibility also works for CaK for you can get a pretty cheap CaK filter from Omega Optical. They have some CaK (394 +/- 3nm) filters selling at pretty cheap price! And just for comparison, Lunt's CaK module is 2.4A while PST is around 2.2A, and the Baader CaK filter is 80A! Therefore, this filter from Omega Optical actually sits between those professional offerings and it's even better than the one from Baader at least on paper.