While the loop part of Velcro is easy to glue with almost anything, the hook part is either polyethylene or polypropylene, both of which are difficult to bond to. Additionally, your Happy Bottom seat is polyethylene foam with a skin surface, which just adds to the bonding problems. Your best bet is to roughen the seat bottom surface and glue it in place with either contact cement (Weldwood Gel is my favorite) or a sealant like Lexel or GOOP (any of the GOOP products will work). Contact cement sticks immediately, but the sealants take time to cure and you'll need to hold the seat in position while that happens. Sand bags or water bags do that job nicely.