Flight Instructor reported an NMAC with what was possibly a UAS during final approach.

Narrative

I was performing the duty of an instructor in the right seat of a small aircraft. Talking to BJC tower; we were making Right traffic for 30L at BJC. Upon rolling out on our Right base I noticed an object off our right wing; same altitude (about 6;300' MSL); estimated to be between 100' and 200' directly west of us; and situated just north of the extended centerline for 30R. The object was roughly in the shape of a 'miniature hot air balloon'. I estimate this 'drone' to be roughly 3' x 3' feet in size. It was red in color and had a distinctly visible bright red pulsing beacon; pulsing approximately the same rate as one from an aircraft. I did not notice any rotors; but did not see it long enough to inspect further. No evasive maneuvers were required based on the timing of when it was spotted. The object appeared stationary in the air.