I would put them as close to the front as you are able. Keep in mind that even that 10x10 size is going to look absolutely tiny on the field. I had a 32'x12' 'cave' prop we made one year and I made the mistake of putting that too far back. It would have been good at the front hash, but we had it in the middle or at the visitor hash, and looked small.

This past year, we did a 32-foot-long, 8' 8-foot-high Alamo and put it 8 steps off the front sideline at the closest side and angled it out to the field about 16 steps, probably. That was effective in my opinion, at least.

Keep in mind that 15' long sounds long, but it's only one five-yard line gap.

I generally believe the 'props' that are just there to be there don't make a lot of sense for the trouble you go through for them if they're not built into the design itself. Do you write your own drill? If not, ask your drill writer for some suggestions on placement. I'd have to see your drill to give you any better answers.