No material is indestructible, no chastity contract is legally enforceable, so how could we make a chastity belt inescapable?
I have an idea. Can't guarantee it would be legally watertight, but...
Suppose, instead of selling chastity belts, a manufacturer leased them out to the wearer; say USD 400 a year for three years, after which the rent would drop to, say, USD 50.
The cunning part is that the device would have to be returned to base each year for checking and servicing.
If it's damaged in any way, then - as per the original contract - the manufacturer charges the full value of the device to your credit card. This could be a hefty but realistic sum, e.g. USD 2000.
Even you would be happy to take the financial hit, the chances are that your spouse would not: "Your bloody stupid fantasy - you live with it until your keys turn up!"
It could even have a timelock, so that there would be no legal issues around refusing to return somebody's keys....