About the size of the text block/ rules. One trick is a different font (some maybe legible even with a different size than the one used right now).
Second trick from my digital layout days is to change the numbers of column - do three columns look fine? (This may lead to a third consideration).
Last, would it be possible to rearrange the contents on Your page? For example, arranging counters on the long edge of the postcard. Not easy, I know, but it is also a way to squeeze as much space as it is possible.
Hope this helps or at least does not give You too much trouble!