If you're afraid that the spacers/ filler pieces would eventually fall out,
you could cut them into the slat, lookingkindo of lime this if layed on its
side:
[[===]]

It would require cutting each side of the slat in order to provide the
filler at the top and an opening between the adjacent slats.
HTH,
Chiz


wrote in message
oups.com...
The simplest solution would be to rout a groove over the entire length
of the top rail. Put in your pickets and then fill in the space
between them with blocks wood thus creating your mortises after the
fact. I would also dowel the pickets in place from the sides rather
than from the top.