I have a couple of ponchos, and have owned something like a dozen more, and it seems to me that all of them hit me just below the knee. Which means pants legs and boots were in the middle of a lot of flow.

My answer was to cut the arms off an old rain jacket, install plastic stiffeners, cut from a bleach bottle, so they ride over the instep of the boots, cords and stops top and bottom to hold them in place, and wear them as knee-high gaiters. My DIY gaiters were free except for the $5 for cord locks. Having the thighs free means the poncho stays cooler during summer rainstorms. When the rain stops I loosen the top of my homemade gaiters and roll them down so they ride on the ankles of the boots so I get more ventilation.

Store bought gaiters or rain chaps would work.