One way to do it is to use two pieces of angle iron where the back of the L is kept spaced out from the other by a certain distance, creating an endless slot..
Example:
I made this way back when so don't make fun (not a fan of my old stuff), but I could have used 1/4" (or whatever thickness you wanted for your bolt thread diameter) flat steel as shims between the angle irons that are butted up to each other and created a slot of nearly any length without any machining / plasma cutting / etc. Considering you need a big slot length (2" or so), this might be easier than finding bracket and since you could make the steel underside function as both structural and your bracket at the same time, I think it would be more efficient than a bracket. And you could way over plan on wood expansion / make the slot much longer than necessary.
Install metal female thread into the table and use a long bolt to fit between the angle iron gap you created with your flat steel shims. The washer under the bolt head sliding along the angle iron's edges.
Or play with the idea with different metal forms (sq tubing, etc), but I think the idea itself will make life easier.
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