I've had a couple of thoughts: The first is: is there any reason why the thread has to be 3/8"? I can see where the length and the width should be more or less the same, but the thickness? Here's one that is 2 1/8" wide by 2" high by 1/2" thick:
I recognize it's 1/8" wide but this isn't exactly an internal engine part, is it? Better a hair wide than too narrow.
My second thought is to find a 2" wide bolt that is too long, then take a 3/8"-16 die to it and cut new threads beyond where the current ones end. I recognize it would be a bit of a pain because it's a u-bolt instead of a straight bolt but it can be done with a vise, a die, and an ordinary wrench. I just did a straight bolt to see how much trouble it would be and got excellent results without much work.
Let us know if neither of these are workable for you and I'll look further.