Cut from larger rolls, thus there may be some arch in the foam as shown, just reverse them and they will lay flat (or close to it) 

These are also cut by hand (on a table saw) so there may be minor imperfections, if this bothers you, these are not the foam squares for you, there are other fine sellers to choose from on eBay.

See pics. 

Custom lengths in 12" width's available, message for details.