I just did this in my Arctic Fox and purchased an exact fit replacement through Crutchfield. First I took down one of the existing ones to get measurements. I didn't want to modify the opening (4 1/2") so ended up with Infinity Kappa speakers that were a direct fit. They needed me to get them the distance between the screw holes, but it really didn't matter with the material our ceiling has. It made a nice difference in sound quality. My radio unit is an IRV33 which leaves a bit to be desired. Next I may add an AMP and Subwoofer which combined will get me a home like sound.