Here's a simple modular missile frigate design (based very loosely on the US Navy's Freedom-class warships).  Self-designated Corsair-class.  It has 4 weapon mount points on its deck, and you can install any of the weapon systems in each of the slots.  Currently included weapons include a gun turret, a single-arm missile launcher, a dual-arm missile launcher, and a Phalanx CIWS.

You only need the STL files, but I've also made available the corresponding STEP files as well as the FreeCAD (RC2) file for anyone interested in modifying or remixing this design.

PRINTING NOTES: You only need to print the Hull STL once (that's the ship).  Then you can print any combination of the weapon STLs.  The single- and dual-arm missile launchers use the same Missile Launcher PLATFORM STL.

The weapon STLs may benefit from higher resolution print setting (like 0.1mm layer height) as they are kinda small and fiddly.  The examples you see in the pictures were all printed on an A1 Mini with 0.2mm layer height, though.

Assembly instructions: The missile launcher pieces should just snap together.  The turrets should be able to pressed through the deck cutouts.  You may get some stringing over the holes during print, but the weapon modules should be able to be pressed through it, and twisted back and forth to free up the rotation.