You have to understand that the + and - posts are different sizes (P+ being larger) and it seems some non-automotive application batteries (which we put in our cars for audio) might not be built in this manner. We've run into Kinetic post being the same size (when they should not be), even some mainstream batteris with a pretty large discrepancy in standards. We are working on a guide for posts on specific brands to make sure what manufactures are within proper tolerances.
If one terminal is so tight it will not fit the post - check to see if the larger opening post (P) will tighten on that post (Do not connect wires, just a test fit) These different sizes are a safety feature so you do not reverse poloarity. Its not something we designed, but something we must follow. If the above does fix your issue, you'll just need a second model of the P+ terminal instead of the N-
edit - and yes, 1/0 and 4 gauge fit direct into the set screw ports on the BT-10P and N. Even our Kolossus Kable.