Make play doh!
One part salt, two parts flour, mix with water until it's right for modeling.
Roll about an inch thick, cover with aluminum foil to keep it from being sticky, sit it on a low bench and lean forward to approximate riding position since the sitbones become more narrow as you lean forward. Remove foil and allow to dry a little. The bones impression will become white first. Measure center to center (preferably in millimeters) and outside to outside. Write down the measurements.
Uh, butt-print mold should be taken, er commando. Or at least with only thin panties or some such.
