sadly you have already passed the part where in my opinion people go most wrong and that is the digging/profile of the pond...i see umpteen ponds for wildlife built with steep sides...they look like a sunken bath or jaccuzi..which is no good for wildlife...i think the problem is when people dig a pond...in their head they picture those awful preformed plastic gold fish ponds from garden centres and emulate that?....
keep it sloping gradual and shallow with beaches and deep in the centre...just like nature does it.
i would go with the sand and old carpet loption....its tricky...yes ok carpet will rot eventually!...but sand only stops the stones it wont stop determined roots and brambles from piercing the liner...old carpet will hold this off for a lot longer...than sand on its own.