There is a formula for size of box & what size speaker to use, but anything will be an improvement over the stock Honeytone amp. Check out my vid where I compare digital & analog recordings using a Honeytone driving a 4X12 cab.
You can keep it simple and inexpensive by finding a used stereo speaker at a thrift shop. Many to be found for less than $10. Or go big and spend some dollars.
But most important, match the speaker impedance to the Honeytone specs, which is 8 ohms.