Is it possible to make an Antimagic condition?

[draco963]

So, my players have come across a Beholder. I didn't think this through in advance, and have since discovered that there isn't an easy way (that I've been able to find) to trigger an antimagic condition on the PCs as they go in and out of the beholder's antimagic cone.

Has anyone experimented with this before? Would trying to hammer this together even be worth the effort? My PCs have a lot of magic items, so trying to track the changes by hand is going to be an utter nightmare...

Thanks!