Somewhere (not sure if a developer question, or Order 66) they asked about mounting ships and vehicles externally, and the answer was to keep it narrative.
Which totally makes sense, I mean you buy minimum 25 thousand credits for that attachment, that is sometimes 25% of the price of the whole ship, you bet that you can get huge and custom modifications for that price. Changing the hull form in significant ways should be an option to integrate those attachments, but players will not always want this. And as we are talking about modifications and customization for ship keeping it narrative is the best advice you can give for something like this.
Right, but the hanger isn't required for carrying craft, you can do that with external clamps, which are a simple narrative option distinct from the Retrofitted Hanger Bay Attachment. External Clamps aren't an attachment, they are just a called out handwave narrative option that allows you to bypass the Attachment mess if your carrier is too small, the fighter count too small, or the attachment is just otherwise in appropriate. Not everyone knows about them though because they are only mentioned once in a breakout box in EotE core, and then never again. So if you only skimmed it, or don't have it, you don't know about it.
So things like the Gozanti carrying 4 TIEs, or a few Walkers can be narratived with docking clamps, no need for attachments or credits. Just boom, done.
On a related topic I suspect that the VCX-100 being statted at Sil 5 had a lot to do with this same issue.