Dual speakers will work well, they will increase the loudness by about 3dB (about what you can hear as a difference). Also, you want to use as large a casing as possible- look at SBS4DCC site for examples. Or, you can print your own custom cases. If you use multiple speakers, make sure to match polarity of the speakers, otherwise they will sound tinny due to cancellation effects. The best shot for most modern decoders is two 16Ohm speakers wired in parallel, to give 8Ohms impedance (about 6.5Ohms DC resistance). You need the space, of course, but you can put them in a tender or a "permanently" coupled box car with wires or a mini-plug to connect the engine to the car or tender.