A clamp on pod filter would work, but it would also be living right between your rear wheels, exposed to everything you ride through. If you're never in mud/water that may be okay.
You're not going to get the stock style filter to stay put without a cage, the cages are unfortunately pretty tough to find used. A guy could make a filter cage out of piano wire, probably what I would do if I was in your shoes.
Hoarder of the finest junk
'82 YT175
'83 YTM200
'85 KLT110
'86 Moto 4 225
'84 TRX200 trike conversion.
'87 Tecate 4
Lots of vintage and not so vintage two wheeled stuff.