Just finished doing that part on a Flipmo3.
It was the hardest part of the assembly process. Prob as its an insulated top and back sides were easier.
Make sure the rubber is good and warm! clothes pins to hold fabric in place on base and to hold skirt up. Unless you got help
Found that a pair of big channel lock pliers worked best to seat and pinch in.
Sure wish the world manufacturing sector would join the future and discover the Robertson head screw Phillips are good for drywall only!!!