Would I be better off building a small cab with a 15 in it, buying a ready-made cab or am I just wasting my time?
Bob,
Although there certainly may be better 8" available, with only 20 watts available you need more efficiency to keep up with amplified guitar, so going with a larger speaker is the way to go.
A 15" will be about 6 dB more efficient in the low frequency range than an 8", the equivalent of going from 20 to 80 watts.
Going with a well braced 1/2" cabinet using a neo-magnet speaker (like the Eminence KappaLite 4015) you could get around 112 dB (one meter) out of your amp with a cabinet weighing around 20 pounds.
You may find a ready-made cab to cost less, and not require any work.
Bring your Pignose to a music store and try out some cabs.
Performance wise a custom cabinet offers many advantages, you can build it to a shape that works best for you, and even build it tall with tilt back wheels and a flip top to store the Pignose in.
Art