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Hi aussiebuilder,
Yes, the reason is, ABS warps more than PLA because of the higher print temperature. Furthermore it doesn't adheres so well. It is easier to print the hull with PLA.
You need to print the hull hollow because you need the gap for the interface.
As I mentioned already in the "Instructions":
The interfaces are supposed to put them fifty-fifty in the gap of each hull part. In this way the the hull parts are aligned and easy to glue.
Before gluing you should mask the edges of the hulls with tape to keep them clean from glue.

I use 2 component epoxy glue. It is easy to handle because you don't must hurry.
The hollow hull don't need to be strengthened further. The interface glued in the middle gives enough stability.