How to enable the decompiler to use given datatypes for better pointer dereferencing

[Wall-AF]

I have some x86 32-bit code that is referencing an array of pointers to, for example, a FILE object (actually it's a union of FILE&_FILEX objects). How can Ghidra be made to recognise this better?

Presently, the decompiler produces
stream = *(FILE **)((int)__poib[0] + iVar2);
where iVar2 is the pointer size multiplied by the index and is stored in the EDIregister to which the pointer size is added at the end of the loop.
It should be able to produce
stream = __poib[i];
as the type definitions should be utilised during decompilation.
Note: i isn't created at all by the current decompiler, just iVar2!

As the plain example above is fairly understandable it's not too hard to interpret. However, once members become involved the decompilation misses them out completely, replacing them with an offset (as seen below)
DeleteCriticalSection ((LPCRITICAL_SECTION)(*(int *)((int)__poib[0] + iVar2) + 0x20));
This should resemble
DeleteCriticalSection (&(__poib[i]->lock));