There is no requirement levied, pro or con, in Rule III (and following) regarding bases. Nick addresses bases under II.14 [and II.15], which if you are not in compliance with, will move you out of the armor categories to vignette/diorama where there are no OOB awards. Under armor OOB specifically, you may block off a visual sight line through the bottom of an open sponson - and I would imagine by extension blocking under/through an engine grill. Add antennas, drill out guns. Use what came with the kit -- no aftermarket guns, stowage, or incidental details. Look what won previously in ShutterAce's link.
In short, a basic base. Just ground cover, no extras -- you're cool
Ed Grune; National Head Ship Judge