To keep it simple, you will need to seed the device with the basic key generation logic or at least directly store them there along with the destination address. Connect it with some form of input, like a button and wifi. Whenever you push the button, say for 3 seconds, you would sign a transaction to send an amount of cryptos to your fixed target address. You can later make it customize the amount.

The benefit there is that if your hot wallet is stolen, then it won't have any cryptos on it until you send some from the warm wallet. If the warm wallet itself is stolen, then all the thief can do with it is send cryptos to you!