My next step was to get a particular woman (already created separately) spread out on that cross with her hands and feet in the correct positions.

I didn't save the exact wording that I tried to use, but it was all variants on something like this:

Woman lies down on the cross, arms and legs spread out along the beams of the cross.

Most of the poses I got back were wildly off. Sometimes the cross itself got modified into a different shape. Even when parts of the prompt request went mostly the right way, the particular model I was using (Gwen Image Rapid Edit) was obstinate about the woman keeping her knees bent.

What got me closest to where I wanted to go was to put the woman in front of the cross, mark where I wanted I her hands and feet to go, then create a prompt (I no longer have the exact wording) basically asking the rendering model to have the woman use her hands and feet to cover the green spots, with the added request that her hands be palms-up while doing this.

This finally got the woman spread out the right way (I'll show this image in the next response), with just a big of still-visible green to clean up in Photoshop.