With my home-made EGR block-off plate (made in 2005), I used 2” long, 3/8” coil from hardware store.

Then I got an EGR block-off plate from Mazda Mitch with the built-in "ear" for the return spring, and with that I use - 7/8” with 5/16” coil. https://mazdaminitruckin.com/egr-plates/

I would recommend to Mazda Mitch to possibly source/include such spring for his EGR block-offs, and adjust the pricing accordingly...