Adapt 5/16-18 threaded rod to 3/8-24
Some DIY ideas...
1
Get yourself a chunk of mild steel or aluminum, some drill bits, and a couple taps. Build a two-sized coupling nut. You probably have a neighbor who has a tap set. You may want to use jam nuts to lock the rods into the sleeve.
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2
Use a piece of C-channel or box tubing. Drill opposing holes for the two rod sizes and attach them with nuts inside and out.
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3
Drill offset holes in a chunk of steel or aluminum bar stock for each rod size. Nut them each from the top and bottom faces.
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4
Use a 3/4&quot; black iron plumbing nipple and caps.  Drill the center of a cap for each of the two rod sizes, nut the rods inside and out, and snugly connect the caps to the nipple.
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