First I got a milk crate and cut out one side as shown here.
This was really hard work with an old bread knife, but I utilized what I had available. A small hand saw or Dremel tool would zip through it in no time at all.
Next I cut a slot big enough for the receiver clip for the harness.
This is the view from inside the crate.
This is the view from beneath the crate.
I set anchor points under his body to eliminate him moving forward in the seat or to the side.
The bottom of the straps come out the sides, once again behind the sides to stop forward motion, they then are attached behind the back to the crate. I set Jack in the crate at this point so I could get it set nice and snug.
Here's the front view up close, pretty snug but cozy.
And finally my handsome boy modeling his new seat on the kitchen counter. I used U-bolts to attach it to my bike's deck and industrial strength zip ties. I will be inspecting the clips, straps, zip ties and bolts before every use. Jack is a pretty big kid at 33lb and 20 months old, and it's pretty snug, especially where his legs stick out (narrow).
I haven't tested this yet but will update with my comments tomorrow after we go for a ride.