Try using the base of the Aegis Quad-gun (or the ring from the icarus gun), a Razorback Heavy Bolter mount (sans
HBs), and mount the missile launcher pods from either a Landspeeder Typhoon or the Valkyrie onto each side of the gun mount.
Did the same thing with my Sisters Exorcist launchers (excuse the paintjob...this was quite a while ago when my fu was weak):
I would imagine the way people were scrapping
ADL kits to get the guns for their Rifleman/Psifleman dreadnoughts their are tons of the bases available for trade.
I have 4 or 5 extras myself...I put the Razorback
HB turret on them and use them for Sentry Guns.