I like piano key borders. You can do 1 inch finished ( if your postage stamp square are that size) of the two colors ( blue and white) then do a 4 inch lenght finshed by 1 wide finshed width of the piano keys in the longer pieces, this would keep everything in the same scale. For the corners do a 4 x 4 square of the small postage stamp size.
Or you can do a first border of 1 inch white , do four wide of postage size squares and finish an outer border (same width of the white)of the blue.
I think its important to work within the scale of the squares. Trying to keep one of the dimensions of what ever you choose no wider than the original postage stamp.