Good advices here, there is no easy cure. Even in professional shipping, magnetic compasses is an issue as various cargoes, change in draft of the vessel, type of cargo among others affects the result. That's one of the reason why gyros was invented, and magnetic compasses are used only as an emergency back up.
One option is to do only a rough deviation table, without any adjustments to the compass. That's easy to update over time. I have never sailed a smaller boat with a accurate magnetic compass and when You needs it most, the heading is going anyway +-15 degees
On paper charts its easy to note the magnetic compass reading when You are doing a specific route in controlled conditions. That is then useful data, but only for that specific course.