Well when you said convoys and then catapult ships, I will inevitably arrive at merchant aircraft carriers.
But let's not stop at WWII era. How would a modern-day MAC look like?
Would it still be spartan-looking like their WWII predecessors, or would modern ship-building techniques allow more-modern-carrier-like appearance (at least the converted parts; if the cargo and its handling space cannot be interfered with, then they cannot be interfered with)? Would the basis-ship chosen for conversion actually be larger? Would the conversion still be limited to bulk grain ships and oilers, or would modern cargo-handling equipment allows for an even-wider variety of choices?
(Yes, I recognise the existence of SCADS package, but MACs are meant to retain most of a converted ship's cargo capacity.)