I initially thought they would have been pretty much the same - at least for the ships equipped with Walruses (so that's Queen Elizabeth and Warspite, and various cruisers, besides the KGVs). Nope. They all used a different design of catapult, and a different design of trolley. I did eventually find one shot of the trolley from a cruiser that looked like it was essentially the top third of the trolley base (so everything from the top down to the beginning of the sloped fore and aft faces), which was enough to give me a clue as to the internal structure of it.
Some of the areas I couldn't make out I have been able to determine via a reverse-engineering approach. It's fairly clear how certain aspects of it are supposed to work, ie how they interract with other aspects of the catapult, so you can work back from that and fill in some of the blanks a little.
I'm currently unwrapping the rails (trolley is already done). After that I am going to start some proper work on the hull textures