Here is a method to build effective valves. They can be used for closed cells (for tips) or closed foils (flysurf foils).
The main idea is to allow the valve to get volume when the air is passing through, but keep a flat shape when blocking the air flow.
Two pieces of fabric are necessary to build the valve. They must be of identical size (on the picture, the red line and the yellow line are of the same length).
I used this kind of valves on my 4.5m² foil, and the tips inflate in a snap (and especially when compared to the 1.9m²). Notice that the 1.9m² is designed for strong winds and the slow inflation of the tip is mainly an advantage for safety reason during take off. For the 4.5m², it would have been a disadvantage.