A reasonably economical freighter is no match for a warship.
But the canonical Corsairs, Patrol Corvettes, and Broadswords are not warships either.
Freighters have a great advantage: Size, and with it lots of Hull- and hard-points. Fill its hardpoints with fixed mounts (that does not cost any tonnage or gunners), and rely on the Hull to absorb damage. Weapons in fixed mounts will cost a fair bit, but will be reasonably combat effective and will not compromise the income-generation potential.
Hangars and Docking Spaces are form-fitting to specific craft.
I would treat a cargo hold as a generic Docking Facility (HG, p60) if the hold has a deck height of at least 6 m [double normal height] (since few small craft has a height of less than 3 m), requiring 3 Dt for each Dt of craft.
I might make an exception to the deck height for small Light Fighters (~10 Dt).
At 1500 Dt you have something like 540 Hull points and 15 hardpoints, far more than a few fighters. Fill it with 15 triple fixed mounts and any standard Corsair would be foolish get into range...
Everything is possible, but I would not consider it easy or cheap to make it practical.
Any DIY solution would be extremely jury-rigged and risk things falling out or jump space intruding into the hold (likely with catastrophic effects).
I would allow craft carried externally with minimum fuss, but that would affect drive performance and streamlining.