Joe, if it were me I would build a fixture that clamped to the side of your 15" beam. The purpose of the fixture would be to house a couple of drill bushings in perfect alignment and perpindicular to the face of the fixture. I would probably install the bushings around 6" apart, which when coupled with your 24" bit should still provide you with adequate depth.

Then I would use a long drill bit and hand drill, using the fixture to guide the bit.

Here is an example of the type of bushing that I'm referring to: http://www.carrlane.com/catalog/inde...173c1b08535246

The fixture could be something as simple as a 4 x 6 that had the bushing holes drilled in it using a very accurate drill press.