Mount a box near your 220 recep, big enough for the timer to mount inside, cut your cable bring it into the new box to timer, bring the other end into new box and make your connections. Box will have to have a cover on it, you can’t put an unsupported unmounted device in a wire run.

better to cut the timer in at the panel as long as it is rated for 220 volts dont know about a model 104r and you have to make sure the timer is secure anyway, sounds like you r trying to take an easy route , best to be safe

Yes, it’s the same as the concept of a timer plugged into the wall, and then an appliance plugged into the timer. You’re just hard-wiring it.

pretty sure the timer needs to have the same ratting as the line in and heater.