I think the rant against the cajon is really misplaced anger--the real problem here, Just a Passing Through, is that you have been engaged to perform and then told you must do it with someone you don't know and have never worked with and will not be able to prepare with.

You are right to be angry about this, because that's not the way musical performance works, and whoever arranged it has no idea what they are doing.

That said, rather than taking it up with us, you should have said all this to the person who makes the schedule.