In the canon (and in boardgames and rpgs) Slave 1 is considered a heavily modified Firespray-31
Basically Boba Fett spent a lot of time and money modifying and updating Slave 1. This would account for it having more hard points than a typical Firespray-31 would for all of the weapon systems Slave 1 has demonstrated.
Within the rules you could either make it a completely custom ship or you could make the closest thing possible based on the Firespray-31 chassis, the choice is ultimately up to you.
The first option highlights the individual nature of the ship and puts the prospects of the players getting their hands on something similar out of reach, however you will be out there on your own and not be able to rely on the Firespray-31 as a guideline. The latter meanwhile might limit the fidelity of your version of the ship, but ultimately provide you firmer ground to operate on.