For me, the Sloppy Seconds from Daddy keeps Last Crusade out of the canon. And on a personal note: EEEEEEWWWWW!!!!
There are some interesting 'Fan Edits' of the Lesser Movies, Crystal Skull included.
While none of theLesser Movies are canon, the novelazations are, so that the choice bits can be taken from them.
A friend and I 'fan proofed' the Crystal Skull novelazation. In the book, Cate Blanchett's charatcer, Colonel Doctor Irina Spalko, had more defined psychic abilities. From that we reverse engineered that the Russians took control of an ancient Romany cult that breeds psychics.