Any reason to not just use the shower? We did that occasionally and use small cups to fill and pour on him when we needed a certain part rinsed. We put adhesive stickers on the bottom to keep him from slipping- like these-
Curious Columbus Non-Slip Bathtub Stickers Pack of 10 Large Sea Creature Decal Treads. Best Adhesive Safety Anti-Slip Appliques for Bath Tub and Shower Surfaces https://www.amazon.com/dp/B075RYQLN3/ref=cm_sw_r_cp_api_i_CfDrFb3NPVJV4