honestly, I just acknowledged that most people did not enjoy scouting as much as I did. my team was of similar size to yours, so we were able to give people breaks & have them work in shifts (usually we'd have the pit crew that was on break help us if we needed). I've known approximately 4 people who are able to scout an entire day without rest, so most normal people (

) will need a break, preferably an hour or longer. your two shift system seems to work pretty well.
at a certain point though, you have to stop giving concessions and breaks to them. they came to the event to work for the team, not goof off the entire time and/or do poor work. Remind them that the scouts aren't just team members who weren't good enough to get in the pit crew: they serve a vital role collecting for you information that the team needs to make informed decisions come alliance selection