Using a fluid bed pond filter has lots of advantages.  However it also has one major
problem up its sleeve.  Should the pump stop for any reason e.g. mains power failure,
the contents of the fluid bed pond filter (i.e. water and filter media) will flow back, under gravity, into the pump and pond through the pump.  This could, at the very least, damage the pump at restart.

My local plumbing merchant suggested that a one way flow valve is, at its simplest, a ball in a funnel and suggested using a 22mm tun dish; I had a quick look at this item and decided it had the makings of a one way valve.

This is the unit as it comes from the plumbing merchants.  It needs  very little work to turn it into a usable one way valve.
The first thing to do is cut the unit into two halves and remove the section between the two cut lines shown on the picture.
Sand both surfaces to remove cutting burs.
On the part that is now the cap you need to remove about half of the outlet casting.
This also shows the cut-outs.
To assemble the final unit, place a hard rubber ball (I used a old mouse ball)  approx 25mm diameter inside the bottom part of the valve, then use acrylic cement to weld the two halves together. Leave overnight to set up (cure) then recoat with cement.
Arrow Shows Normal Direction Of Flow