Line the tub with a single sheet of HD black plastic. Then set in some PVC pipes for anticipated electrical runs. Then insert some boat tackle storage boxes with flip tops, then set in a few different sized pails with black sheet plastic around those. Add anything else you want to ‘mold’ into the tub as you will then fill the voids with expanding marine foam. The buckets can then be removed and you will have recesses for the real bait and fish pails you will use.
Flush trim the top of the foam and cover with light plywood or some other light decorative top. Mold in rod holders, battery boxes, wire in usb ports for charging things, mold in a spot for your heater, etc.
Why line black plastic in the tub before you pour? Yep, so it will all lift out for drainage or if you want to pour a new mold system another time.