2 inch, 500 Hz rectangular horns?

It is going to be big! (and even bigger in 60° version, if you want horizontal directivity control down to 500Hz)

JBL current offering is meant for 1.5" drivers.
If you are not afraid but the size (and depth!) the 2360/2365 (especially the B versions) are good candidates.

If constant directivity is not mandatory the TH4001 could also be a candidate.
 
Horn DIY

Are there any outstanding, off-the-shelf, baffle mount, 2 inch, 500 Hz rectangular horns out there?

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Horn DIY

If you relax your mounting requirements and build it yourself, you will get superior results for the money and time spent. See what is possible at the following website [1]

Regards,
WHG

[1] Yuichi’s Audio Lab
Yuichi's Audio Lab.