3 tried and true methods in ease of use order;

it may sound overly simple, but take a clothes hanger cut both straight parts just past where they turn up from the bottom, bend a hook shape in each of the cut ends then form the wire into an arch that is quite a bit wider than the bait, clip one of the hook ends into the front of the bait and the other into the tail section on the pin holding the tail or on the rear hook hanger. You may have to experiment a bit to get your hanger wide enough to fit the bait but the idea is it wants to spring out to straighten out the bait and thus keeps tension on the bait and the joint open. I have used this method to paint BBZs and a number of other jointed baits.

OR if you have LARGE C clamps used in wood working etc. take a wire and tie the nose of the bait to the fixed end, and a second wire to the tail of the bait and wrapped around the clamp screw. Then open the screw handle until it puts tension on the bait. This can also be done with the sliding clamps if they are tall enough, just reverse the sliding part on the handle so that when you squeeze it puts tension on the bait.

If you dont have a clamp large enough, cut a wide enough and deep enough C or wide U shape out of 3/4 in plywood and create a 'clamp' using a long 1/4 20 screw and a couple of nuts to hold the screw in place on each side of the pilot hole through the plywood and keep the tension steady put a nylon bushed nut on the end to secure the wire in place when it is wrapped around the screw. A bit extreme but definitely reusable and effective.