Insulin Dose reminder

Many people with diabetes sometimes forget whether or not they have taken their daily dose of long acting insulin. Since I'm one of those people I made this simple reminder.
You fill it up with pen tips on Sunday and then dispose of them after using. If that day's pen tip is still in the holder, then you haven't taken that day's dose.
I designed it to fit the Lantus Solstar prefilled pens, but it should fit any pen that is approx 16mm diameter and up to 172mm in length.