A modified turnbuckle with eyes bent at a 45 degree angle, supplied with pan head wood screws. For retrofitting existing wooden keyed stretcher bars and braces to make them expandable and contractible. You will need 4 in order to adjust all four corners, plus one at the two ends of each brace. Another attribute of the turnbuckle is that it acts as a corner brace to stiffen corner joints.
Flat rate $5 shipping, or get free shipping when you buy ten.