Had to go with different sections/shapes than originally planned because of the size of the piece of perspex I had.
Then put and inch square length of wood in the bottom along the front to raise the panel up and create a slope:
Put a 2" wooden block at the back corners to create the slopes from there too:
Seems to fit as it should!
This is the drainage fitting I chose to use:
Take plastic off perspex and glue down onto wooden blocks and baton and put plastic plug fitting into drainage hole:
Had to put another piece of perspex at the back to allow for the depth of the plug 'lip'
Then line the whole lot with pond liner:
Got some gorgeous foliage decor paper to line the sides and back so all waterproof (Make sure this overlaps the pond liner at the bottom so any run off ends up on the floor, not down the back of the liner)
Then popped the plughole onto the fitting (all sealed with pond sealer underneath) to complete the drainage system: