It is not possible to actually 'seal a private key'. First of all, it is basically impossible to safely create pre-funded collectibles (safe as in: the creator has no access to the private key).
The method Leo mentioned, allows you to make user-funded collectibles safely, but that kills the coin's collectible value, as now the user who funded it, can scam a future owner of the physical coin.

In my opinion, for substantial amounts and as technology advances / gets cheaper, hardware-wallet-inspired collectibles should be considered.
For instance, it is today possible to build a device which uses an open-source 'avalanche' circuit to generate entropy and private keys and store them in a secure chip, which even the creator cannot extract, although he has hardware access to the device. The device would be able to display a Bitcoin address, though, and destruction would be necessary to spend the funds.

This is all possible and mostly how a hardware wallet works (only really changing the way the secure element grants you signing access), but those aren't exactly cheap, so this is the one drawback.