Well, for the base (for when I give the dinosaur away) I found a styrofoam lid.
It comes from a foam medication box - my neighbor put two out with the rubbish and as soon as I saw the two lids I nailed them!
The plan is to weight the base quite simply with a layer of stone chips (from the garden), and then do a spot of landscaping.........
I propped him up with a box - without the box there is plenty of space for him to move in and be centered. I will probably mount him diagonally - always looks better in dioramas then aligned with an edge.
As I haven't found a deserving home yet for the dino, I will store the two together until need - takes up less space in the cupboard!