Easiest Way to get an "Aux In" On an Amp Without One?

I enjoy having the metronome and/or backing tracks I'm playing along with be to be LOUD and come through my cab. The little metronome I have isn't quite loud enough. On amps with aux-ins, I just plug the metronome right into the amp. Buuuuut I have a few amps without this feature. So I am trying to figure out a convenient way to get what I want with these amps.

I'm aware of the Walrus Canvas Rehearsal, which I am really strongly considering. But just wanting to see if there are better ways than spending $200 on a metronome :), so hoping there is a cheaper option.