Very nice bag, I like it a lot. Well done.
Not sure how big of a knife you are talking about. The knife I sometimes hang off my bag has about a 5" blade. One thing I've done is to make a welter sheath leaving just a small tab coming up from the back (like you've cut the belt loop on a standard fixed blade knife sheath), the table needs be wider than the strap.
Then I cut 2 slots horizontally across the tab, just wide enough for the bag strap to squeeze through. Unbuckle the strap, thread it through the slots, and the knife will hang at the back of the bag.
I've been toying with the idea of a small rectangle of leather sewn at either end to the back of the bad, horizontally. Wide enough for a center seam sheath to slide into between the bag's back and the rectangle of leather, using the same folded over flap of leather that keeps the sheath from falling through one's sash or belt to catch on the strip of leather.
Hopefully I explained the 2 ideas okay.