Baby bath time!

Ka
my 5 month old son absolutely loves bath time! However, he has gotten way to long for his summer infant bathtub. I have tried laying him flat in the tub with a towel under his head and filling it just below his ears but he hated it. He loves to kick and splash and that doesn't really cover his feet to do so. Any recommendations on what to use until he is fully sitting on his own? Thanks!