Burnsco smart marine have them in different sizes...
Range from around $25 thru high 30s
I made mine out of a spare of vynl left over from a car project....sheet 800x 600mm that happened to also match the grey colour of the boat seats
Folded it over the cover.. spot taped like spot welding, to shape nice slip on fit...then trimed off the folds inside and around the base of the cowl...taped up the seams with some duct tape.
Thats Mark I MarkII with be getting around to sew the seams.
found that the size required for a neat slip over fit for a V6 johnson ( 500w x700L x 600 to 700mm H)...on the brought ones would be baggie in a couple directions.
Althu the boat is under a sail cloth cover, driving rain can still get in thru the cowl air vents and pool around the bottom of the powerhead inside.