Well, one could try two-channel equalization in an attempt to compensate for the differences. But alas you are due for a different pair of matching speakers.

Wel, the purist in me says "Fuhgetabudit!" but I get that Free is free and you need something to play the tunes.

One BIG issue is that the larger speaker will have a much different load on the amp, causing any number of odd things to happen to the sound (no dangerous or anything though). You may want to try really thick wire on your smaller speaker to increase resistance, though even that will do only so much. Likely the smaller speaker just is, (or to your amp will appear), much more efficient than the larger one.

If the tower has a port I'd stuff it with some cloth to keep the bass overhang at a minimum compared to the small speaker.

Lastly, If you can get the smaller speakers tweeter to be the same height as the tower, you will go a long way to making the setup gel.

Good luck!