Starship Modification Limit
In chapter 4 of Starships of the Galaxy, the Modification Capacity by Ship Size lists the maximum amount of modifications normally available to ships. Following this is a method for creating additional modification slots for a given starship, involving additional costs and heightened installation DCs.
Some tables might find this daunting, as it means a ship can never be "done" and has no upper limit on how many modifications it has until the DC becomes functionally impossible to achieve, though players can still technically attempt it even then. Or a table may simply find it too complicated to keep track of. Whatever the reason, consider omitting this ability to create additional modification slots to ships, and have the table represent a hard limitation on how many modifications a ship of a given size has. (Barring use of the Equipment Room suite, which effectively consumes a suite slot to add 3 more modification slots.)