Do you have a sheet she can wear? There is a rig you can use with a blanket or sheet, a jug and some netting if so. There are actually several ideas online....just a matter of finding what works for you. Some have luck loading/unloading the mare on a trailer to trigger peeing. It's really a matter of knowing your horse and her triggers and habits. When we had a mini we wanted to test I just ended up spending an hour with her on a lead in the pasture and when she needed to go I was able to catch a sample. Our th would be easy because I know when we go to feed she runs into the barn to per in the fresh bedding....every time.