Update: I ended up getting a pair of these from Dover because they were on sale at half price: https://www.doversaddlery.com/ds-madison-field-boot-black/p/X1-381465/
So far so good, not as comfortable as my non-zip boots though. The zipper at the back is stiff and bothers me a little but for the price I couldn’t say no.
Thanks to everyone for the advice!