I have been using this one for about 6 months and it has been doing a good job for me, ideal for 540 size rotors, the larger 4-pole 1/8 rotors are too powerful magnets and will max out to not get readings on 1/8 rotors:

https://www.amazon.com/gp/product/B073W7MPNN/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1

https://www.amazon.com/gp/product/B073W7MPNN/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1