Here's my solution, but it's not an astromech:
T-65R [TORPEDO] (X-Wing only) {-1}
The ship’s upgrade bar gains the System Upgrade icon.
Fluff: the T-65R was the recon variant of the X-wing; it replaced the torpedo bay with an electronics suite (per the X-wing books).
The problem I have with this, and other "slap on a system slot" solutions, is that it basically melts the X-wing and the E-wing into almost identical design space. Let's not fix a ship by stealing another one's thunder.

