Not sure there isn't some mixup in terminology here.  Your subject line says "hip braces", but you are talking about butt blocks and thigh braces.  First, butt blocks should be pieces of solid wook, most likely to back up the butt joint in your deck panels.  Thigh braces:  Not sure if you are talking about foam pads to go ahead of your seat to support your thighs and knee braces that go on the underside of the deck at the edge of the cockpit opening to allow your knees to lock you into the boat.  Carefully review your building manual.  There are most likely pictures of these items that should clear up many questions.  Sorry I cannot be more specific but I do not have a C17 manual.

Hip braces by the way go on either side of the seat to keep you butt centered in teh boat.  they are a piece of plywood on each side running from the underside of the deck down to the bottom and are faced on the inboard side with foam.

Good luck

Paul G.

My (very old) blueprints for 17LT call the plywood braces on either side of the seat "thigh braces".

Those sound like hip braces to me.

Paul G.

My LT-17 plans are dated 1998, and they also refer to the plywood braces on either side of the seat as "thigh braces" but as Paul G. notes they are really hip braces.

Also, as Paul G. notes, the "butt blocks" are blocks used to reinforce the joint where the forward deck panel meets the aft deck panel.  From the top view on the plans they appear to be adjacent to the seat, but that's only because the plan represents a three-dimensional body in two dimensions. In fact, the butt blocks are actually on the underside of the deck.