My golf cart seat is 40" x 20". The arm piece is 12" centered on each end of the seat. I like the first one on the pinterest site that sync posted above and has the loop over the arm rest. Several of the ladies have this style so they can easily take it in the house after golf. I have not made one as I have a fitted seat cover but it is so dirty from the dog that I need a cover for use when I golf. Don't make it too tight as it needs to be bigger when 2 bottoms sit on it!
HTH