Quoted:
I thought about zip ties, but I'm using a borrowed helmet, so I might as well buy my own that's more purpose driven to use in ... other situtations.
Well, you can either use a bike helmet in "other situations" where it is entirely inappropriate and could be the cause of serious injury.
Or.
You could use a helmet designed for "other situations" where it is entirely inappropriate on a bicycle and could be the cause of serious injury.
Right tool for the right job. A lot of science goes into keeping your head safe based on environmental risk factors. Screw with it if you want, but don't be surprised if you end up with undesirable results. There's a reason you don't wear a bicycle helmet on a motorcycle, and wouldn't wear a motorcycle helmet while rock climbing.
Your best bet is to get something like a Protec or PT Alpha. Both of which are designed for skateboarding and snowboarding where whacking your head on the concrete generally occurs in a different manner from that caused by a bicycle accident. They work fine as a general bump helmet.