check out Northcott Fabrics website--they have a series of 4 panels that are perfect for a little boy play mat--2 city/roads, ones an airport and the other a harbor and they all "fit" together.
Also, I've made round,reversable ones with my 10 degree ruler--if the child is older you can run a cotton cording through rings on the outside so that they can leave toys in middle and pull the cord to become a closure/handle--but do not do that with little kids that could get hung up in cord!