How to get Ethernet cable into a small enclosure
Use cable gland, they are for cheap and water-proof (IP54 to IP67). Try PG11 (5-11 mm dia in), with or without a little forcing/hack (depended on concrete manufacturer/model), it is possible to trace an RJ45 pre-terminated UTP cable inside the enclosure.  Mounting a PG onto a box is simple: only round hole is needed. Drill it on a free surface of a box side and install the PG.