Your idea of a stop collar on the post isn't a bad one at all if the other solutions don't pan out.

But before you go with that option try replacing the seatpost clamp with one that uses a bolt instead of a quick action lever. It's MUCH easier to generate serious clamping forces with a bolt as opposed to the lever style and that may be all you need. For longevity of the clamp threads try to get one with a bigger bolt size instead of the small road style clamp bolts. And be sure to grease the threads so you can achieve the clamping tension with lower torque values.