Try leaning forward and tilting toward the right, this way the left arm can extend behind without rotating at the shoulder, grab the belt between the seat and B pillar, bring the hand forward bending at the elbow while moving back in the seat, then reach across the waist with the right arm to get the buckle.
Hey...... I use one of those when I'm at the bar and the guy 3 seats over just got served his brew and hasn't taken a sip yet.:beer
Simply brilliant!!! :beer And to think I was going to suggest getting in through the hatch & grabbing the seatbelt on the way to the front seat.