Camp site in NST ?

I'm going on a little bike trip next week - riding over from Phang-Na to explore - I rode through on the 4224 / khao luang / reservoir area before and it looked nice for a return trip.

Having been recently exploring the logging tracks in Malaysia recently, I find it easy enough to just throw up a tent in the late afternoon .. Convenient and also free.

I have seen tents on the beach at Kanhom before - is there anywhere else inland where I wouldn't be troubled - I feel begrudged to pay national park price when the forestry commission or nature conservancy areas are free and also I find the staff friendly.

Any ideas for camping or hidden attractions to visit would be welcome.