We put this together a few weeks ago and noticed it started to bow at the ends.  My son decided he wanted to build it himself for a school project, and I took it apart and bent some of the pieces back so they'd lay flat.  Now that he's about a 1/3 of the way into the build it's bowing again.  Any tips to avoid this or is it just something that comes with these wide builds that don't have a solid base?

I assume you are talking about it curling up at the ends? I would just finish the build and make sure everything is tight (push it down flat slowly but with force every once in awhile) and see what happens. Build on a hard surface like a table rather than a carpet or rug. Good luck and welcome to Brickpicker!

Yeah, that's what it is, curling up at each end.  He is building it on the kitchen table.  I just think it may be how a set like this ends up as you build the base vs. it being one of the big one piece bases.  That, and then all the weight ends up being more towards the center of the set.  I'll keep trying to push it down without damaging the build.  Maybe the two weights I found and put on each end this morning will help.