Suitability for which dinos, just the t-rex's?
Here is a suitablility guide:
60% grass
25% gray stone
10% decidous
5% water
3% rocks (fill entire square)
7% brodleaf bush (dont use that other grassland tree, use the bush and fill entire square.)
You can also add some elevation, but don't need it to get 100.
And like Ouch said, lower the exhibit into the ground 2-3 squares then use smaller, normal animal fencing. They cant jump out and it will save you money.
If there is any other dinos you are having trouble with check at
Wisco's