I think the top holes fit a 1/4" bolt with 20tpi thread.

Try these:
[Link: amazon.com]

Or these if those aren't long enough:
[Link: amazon.com]

If you have a Lowes or Home Depot nearby, you can stop in and buy 1 bolt instead of ordering a whole box first. And make sure it fits properly.

[edit]: looking back on this thread, yeah you better buy just 1 bolt to check to start. 1/4-20 is pretty common and surely someone would have tried that by now, so that may not be it.