I built a simple venturi skimmer. It is an in sump model.
I don't have any plans for it. You can see from the picture though...it is a 6"X9" acrylic box on bottom. 4 1/2" acrylic tubing for the riser and collection cup. The hole in the top of the box has a 1" bulkhead in it. Then is attached a reducing bushing from 1" to 1/2". Attached to that is a venturi valve, and on top of that a 1/2" barbed fitting to attach the pump. This thing skims better than anything I have bought and costs way less to build.
On the bulkhead on the side of the box, there is a 1" gate valve to control water/foam level in riser tube.