In the plan there are no flaps. It would be really nice to have flaps though, so I looked around a bit to see the ‘how’ of it.

This page from www.boldmethod.com is instructive on the kinds of flaps.

It shows the four different types of flags: plain flaps, split flaps, slotted flaps en fowler flaps.

There is also a nice youtube video that shows these flaps (and more) in action: video

As this picture shows, the CL-415 uses slotted flaps:

(Image from seawings)

The hinges are external and the flaps are pushed backwards when deployed. It should be entirely posible to use the same approach for the model.

The flap itself is a kind-a wing in and of itself, though it has a slightly different profile, possibly a little more circle-shaped where it touches the wing (when not extended) to facilitate a better fit with the wing itself.

Another video mentions that 30% of the flag should be located under the wing (when not extended). Since the flaps and the ailerons on the 415 have the same depth (as seen on-top), that means that the lower part of the flaps should protrude into the lower wing. Which needs to be changed to accomodate this. The lower hind-bar in the wing must then be moved forward, or maybe it will be possible to replace it entirely with material used for the new shape.

(Image from seawings)

This picture shows the hinges and we can see how the center of rotation is pushed back to be under the flaps itself. It seems to be placed just behind the line where the ailerons meet de wing. (Just follow the rivets)

The way the hinges are constructed seems to suggest the use of (Robart) pin hinges in the model. Another item for the parts list.