My kid brother, who's got a fresh PhD, an MD on the way, and two kids of his own, pointed me to this soft foam bath cradle, which they've found to be very useful and comfortable. It looks like a molded baby yoga mat with drain holes. It has a gimmicky-sounding temperature indicator, which I'd rather not rely on. (Test the water yourself with the back of your hand; if it's at all warm to your touch, it's too hot for the kid. The AAP also recommends turning down your hot water heater to below 120F.)
Best of all, it's small. The last thing I wanted was some extra baby bathtub contraption cluttering up the place. And it's only around $6.
Buy the White Hot Infant Bath Cradle at Babycenter, $5.35 on sale.