I know you can use a round SPS under a square cake board, but will it still be as stable if you do a rectangle?  I'm doing a rock climbing wall cake and I baked square cakes, but I'm thinking it will look better as a tall rectangle.  It's going to be picked up and transported by the client though.  Should I leave it square or will it be okay to do the rectangle?

Thanks!

Oops!  Forgot dimensions.  If it's a rectangle it will be 8"x6" with a 6" round SPS plate after the first 4 layers of cake/frosting.

AIt will be fine. The cardboard will support the extra 2" .