You are right Tim, a good triplet is a better choice for narrowband than the same aperture good doublet, mainly because the triplet can be faster than a doublet to produce the same or similar quality of data. However, we are discussing budget setup and doublets can be very capable of providing good data.
One example of decent narrowband data (and mediocre processing) captured with a doublet can be found here:
https://www.spacetelescope.org/proje...r_lipinski_03/