Mary (and the other ladies),
There is drop-seat zipper available on the ladies' Kokotat front entry suit. It is a horseshoe shaped flap with a zip that starts at the back of the thigh, goes up to the waist area, across, then back down to the back of the other thigh. If you look closely at this picture, you will see the end portion of the zipper.
The drop seat zipper feature, available only on women's suits, cost about $160. One or two uses and you're happy to have paid it.
Here is the link to the womens suit:
LinkThere is another, more challenging option that requires a front relief zipper and the use of a special urinating funnel tool that is ergonomically shaped for the women's anatomy.