This post describes an iPhone app that allows you to measure the magnetic field
Facing the same problem you did (in fact, probably worse, since my deck is also electrically powered) I ended up with a large mast. https://photos.app.goo.gl/rrNTHjFD2JTFJ7Kt6
The mast presents operational challenges since it prevents the mower from mowing under trees. I’m working on switching over to a dual GPS setup and using yaw from the 2 GPSs. Several threads on this forum about that including Use Heading from Dual GPS - #53 by Christopher_Milner