Hi Bradley,
this construction is a master for make copies, no glue only every wall is suported by another wall. fountain must be simetric inside the arch
Mike, I use polistirene plastic , same of Evergreen but I buy in industrial store, this plastic is used for make commercial sings and is sold in very big sheets 1.5X2.5 mts, of course you can buy also only a portion of the sheet:-)
hope that this helps