It's an odd concept to try and model,as the whole point of one is a Trojan horse with teeth.so you stuck with a merchant man,or a merchant man covered in guns.I just go with a tramp steamer and removable deck guns.Though this is more of a coastal fishing boat that can also be changed to look more like a puffer,(Ive no room for a Q ship any larger.But they came in all sizes so it works for my needs)
In smaller scales I'd just use a merchant ship model as it is just that until the last second anyway.
Mark.