Wearing last tray as retainer?

Lost my top invisalign retainer. Been wearing my last tray as backup for the last 4 weeks or so. Wondering if this is something I can keep doing or do I need to go back and get a new retainer? I had some unexpected expense this month so honestly if I can avoid forking over 300+ dollars I will.

I wear it for about 16 hrs a day ....

Thoughts?