The upgrade project of the 25T-CSM to 30 T is on-going at HFLSM, IMR, Tohoku University. The goal is to obtain 30 T in a 32 mm warm bore by replacing the present Bi2223 insert coil with one wound using REBCO. In order to develop a robust REBCO high field insert, we adopt a two-tape bundle conductor with a face-to-back configuration. We confirmed that this conductor configuration mitigates effectively the risk posed by local defects in an R&D coil: a stable operation was achieved with negligible degradation in coil Ic even with a severe local defect on one of the two tapes. Quench protection from possible hotspot in the REBCO insert using an early detection method is made possible by the two-tape bundle configuration and the thick Cu stabilizer. Other R&D coils using such conductor were tested with large hoop stresses up to 460 MPa with no degradation. On the basis of these R&D studies, a robust REBCO insert is designed for the upgrade. The designed REBCO insert has a capability to achieve 35 T with enough Ic margin if we accept a maximum hoop stress of 500 MPa.