We bought a used playpen that was missing a T-shape joint.
similar to this one:
https://www.amazon.de/JUSONEY-Baby-Playard/dp/B0BJZNLFTJ?th=1
So I designed one in Fusion and printed it in PLA-CF.
came out perfect and fits better than the original.
I attached the file as f3d and STEP, so it can be customized to suit your needs.