I went with a rate gyro compass
(Maretron SSC200) on my steel
boat, and hesitated somewhat when placing it in the fiberglass
instrument pod over the companionway
... since that gap in the steel deck
introduces a scary 35-degree variation when moving up and down with a hockey-puck compass. Reassured by Maretron, I went ahead and mounted it, then ran through the calibration by doing multiple controlled circles at a constant rate of turn (see below for the track... there was a little current
running in the channel). Astonishingly, it's now correct to the degree, perfectly tracking the binnacle compass that has been professionally swung.