put the middle slab in the water(?)
three-tall upper cobblestone walls, two-tall fences
upper and lower slabs on the top and middle cobble walls, in the corners (so six pairs)
cover over everything but above the cobble walls and fences (so empty space over cobble walls and fences)
the roof is the level over the three cobblestone walls
the slabs are preventing the pairs of villagers (on each of the fence's four sides) from interacting
drop villagers onto the fence, push them in relevant direction
2 south - 2 north - 1 east - 2 west - 1 east
remove top fencepost, so now the cobblestone walls are two higher than the one fencepost
have all four cobblestone walls AND the fence covered with bottom slabs (so five) instead of what was covering them already
in the layer over the bottom slabs that are covering the cobblewalls, do a glass in the center,four top slabs
five pistons over those, push the blocks down twice so the slabs in the previous two lines meet up with the slabs from the
  ...third line
the three cobblestone walls are now in the air layer between the bottom one, and the next two layers up
...so the top one is in line with the one fencepost
remove the bottom two walls

food water stream:
slime blocks in the corners at the level of the now one-tall fencepost
ice blocks to the right of them
top slabs over the ice blocks
water flowing to the left, over the slime blocks, stopped by the slabs on the right
the channel formed by the top and bottom slabs that the villager heads are inside -- those layers are where the
...blocks above and on the sides of the water are in
I guess leave a hole over one of the slime blocks? in the layer the top slabs are in. The upper top layer. The roof.
Not the lower top slab layer, which is separating flowing water but not stopping items from traveling over ice.