Could try getting a custom made strap and request for a thicker padding near the lugs where the spring bar is inserted.
Might not totally get rid of the gap but it would probably reduce it
Not quite, the padding will not reduce the gap. What you need is something like one of the "Curved End Hirsch Straps". I owned one myself and can highly recommend it. http://www.hirschstraps.com/collections/curved-ended-hirsch-straps