I am running 250-250-250-3/0 SER from an outside disconnect to an indoor breaker panel and want to make sure I am properly securing the cable where it enters each box. The problem that I am running into is that the common sizes of clamps (like the one in the link below) are too small. What is my best option?
That's because that clamp is designed for flat-ish cables
Your problem is that you're looking at a clamp designed for flat-ish (SEU) cables. For the SER cable you're using, you need something designed for round cables instead -- you should have no trouble getting your cable to fit into an Arlington 849S fitting, for instance.