I am making an outdoor garden bench. Originally I was going to use a
straight top rail for the back, and use separate mortises for the slats.
I have decided to curve the back top rail, as I think it will look
much better.

Does anyone have a suggestion as to how I could mortise the slots to
accept the slats for the curved sections? I thought of routing a dado
along the bottom and filling in the spaces with wood spacers between the
slats, but I don't know how that would work in the curves. Wouldn't I
need to take into account the curve angle for the spacers? I don't know
how to do this.

Can anyone help?

Thanks.

Chap