If you have access to a drill press and a tap set - find a piece of bar stock, aluminum or brass, steel, whatever, size depending on how big a clamp needed. Example, aluminum bar stock 3/8" x 1/2", cut two pieces 2" long. Clamp together and drill through one end with a # 7 drill about 1/2" from one end. Separate the pieces, open the hole in one piece to 1/4", and tap the other with a 1/4-20 tap.
Cut a piece f 1/4-20 threaded rod about 4" (longer if necessary) long, and screw one end into the hole you just tapped in the one bar. Let it project out about 1/4", and tighten a hex nut against the bar. Add a hex nut to the other end of the threaded rod, then slip the other bar with the 1/4" hole over the rod. To use, insert the bars into the space you want push apart, then holding the bars in position, screw the nut towards the bar with the 1/4" hole, spreading the bars. To work well, the bar with the 1/4" hole must be thick enough that it can't tip on the rod.
Scale the sizes up or down as needed. By reversing the bar and nut on the end with the 1/4" hole, it works as a clamp also.
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