I grew tomatoes in pots this shape & approximate size outside on year. For me, it was a succession of improvised supports over time.
Knowing the variety would help. As would knowing what's above that you could anchor string to.
I'd also recommend, since they're inside, removing the suckers to keep them form getting too out of hand, space-wise. But I'll defer to the tomato experts here!
"No crime is involved in plagiarizing nature's ways" (Edward H. Faulkner, 1943, "Plowman's Folly," University of Oklahoma Press).