I realise that this project doesn't meed the usual high standards of some d-i-yers here. However I thought you'd be amused if nothing else.
Here's my solution to the problem:
Just visible under the seat is a black plastic pipe with about 50 holes in it for flushing. The pipe goes outside to a tap. The tap was rather stiff so I attached a long lever to it, and a cord that goes through a hole that is pulled for a flush. Note the knot in the cord to limit the length of pull. There's another knot inside.