To get a sense of how useful a square rabbet can be, check out this video of Bickford.
Matt also did a nice tutorial that I followed pretty closely.
I decided to make the plane out of a piece of quartersawn jatoba I had. The sole of a rabbet takes a beating, so you either need to make the plane out of something hard, or box the sole.
Before starting the plane, I made a simple saw/chisel guide. These are extremely handy. This one is cut to 55° on one end and 65° on the other, with a groove down the middle that fits over the plane's body.
I started by making two cuts with a backsaw to define the throat, using the guide.
Then I drilled a 1" hole with a forstner bit, and knocked out the waste.
After that, I drilled a hole down from the top, then used chisels and vixen files to refine the wedge mortise. I carved the escapement with an incannel gouge, then refined it with a file and sandpaper.
Here's the finished plane:
I almost forgot: The plane is left-handed! When you live in a righty world, making something specifically for lefty use is always great.