Is there a way to prevent uke from rolling out of a shihonage pin? How did shihonage become a pinning technique anyway? Maybe it isn't meant to be a pinning technique?
During the drop, do not step forward. Just drop vertically on the spot. The feeling is like Osoto-otoshi.. i.e., the victim ( I mean uke) will have a feeling that his legs are swept under him.
Assuming the ground is solid asphalt and not padded mat, this will prevent uke from rolling out and also doing any other other physical activity for a while.
If there are padded mat... then following advices from post # 26, 27 & 28 are also fine.