I am looking for brackets that will mount on the rails to keep the
canoe from sliding side to side. The rail shape is NOT round or square,
it's () , turn 90 Deg.

Sounds like Yakima Mighty Mounts might work. You buy the size that fits the
Sienna's bar. They are really made for attaching bike racks, etc. to oval
bars, but I use a pair on my front bar to keep the canoe straight, and to be
able to snug it up with the bow line. They function like cheap gunwale
brackets. I think the correct type is called a vertical Mighty Mount. They
stick up above the bar an inch or two. My rack is only wide enough so that I
can do this with a solo
canoe, width 30". I pad the Mighty Mounts a little, to protect the
gunwales.

Another problem is the front tie down! They don't have a bumper up
front. Only plastic parts, that would bend if I tried to wrap a rope
over them to get to the underside.

To tie the bow line off, with no real front bumper, people mount short loops
of straps that stick out from inside the engine compartment, between the
hood and fender on each side.
I've seen them sold commercially at Canoecopia.

Jim