Ya on the warping. Building grade 2x4 these days are still sap wet.
If you can find some old hard ones that came out of an old house they would be better.
Another option I seen done here. Some one cut 1” x 4” strips out of bigger Popular boards. Then glued
2 of them together to his spars. They tend to hold each other together and stop most warping. Just watch
out you join them to the side wall. You don’t want to drive them apart with a screw in the glue joint.
This page ( half way down ) show the spars and later how they are installed.
http://www.campingclassics.com/stripper4.html His homepage and build logs. for later reference. + he sells a nice guide for building.
http://www.campingclassics.com/index.htmlKeep us posted.