I just make a square (or the same footprint as the base board of the wendy) by scewing four pieces of structural timber together (or you can use braces like these from Aldi or similar to secure the corners
http://aldi.co.uk/uk/html/offers/special_buys3_20318.htm
Then I simply add cross members going from one side to the other by putting them inside the frame and screwing from the outside (bit like a fat ladder). The braces need to be going the opposite way to the barers on the existing wendy house so that they give extra support (by forming crosses where they touch).
I shall (weather permitting) be making a base over the weekend for our new dog kennel and will take photos as I go.