[Lord Grey Boots]

Lets see,

Rambling stream of consciousness thinking....

Using thick very tough plywood, like appleply, tempered glass, and shatter resistant film on the glass, you could come up with something pretty tough.

Back up the shatter resistent film with a steel grid behind the glass.

You would need to visit one of the specialty plywood suppliers, and be prepared to pay a lot per sheet. You won't find the wood you want at your local Home Depot or equivalent.

Use hidden hinges, and have the opening framed so that even some one gets a crow bar in the gap, they can't pry in a manner that stresses the hinges.

Add reinforcing strips on the inside of your panels to keep them from being flexed by someone with a prybar.

Bolt the sucker to the wall/floor with bolts you can't reach from outside.

I'd also add a local alarm.

Reinforce the opening with steel. Even better, start with a stout steel cabinent, and sheath the outside with the tough plywood.

It will cost more than you think.