Over 15 years ago a rider at Transworld Motocross fashioned something on a KX250 with pieces of a crossbar pad held to the seat with duct tape...
More recently, Dean Wilson created a seat for riders to grip the bike when standing and Blake Bagget's designed a seat bump to prevent rider from sliding backward...
This year, Ken Roczne and Hunter Lawrence are racing with small carbon fiber frame attachments for their boots to grip the bike.