In article ,
Winston wrote:
The proper answer is 'buy or borrow a table saw with a dado blade'.

However, I don't want to.

I want to use my plunge router to place a 3-1/2" square lap joint
about 3/4" deep on the ends of several pressure treated 2x4s.
I will don a respirator first, no worries.

I've designed an machined jig to allow me to do
this but I want to know how you pro's would approach this
challenge before I search my junk pile for parts.

My Google-fu was not up to the challenge. Instead, it revealed
some table saw adapters.

My band saw refuses to cut anywhere near straight so that's out.

First prize answer would be an inexpensive jig I could pick up
at my local hardware store.

What would SuperWoodworker do?

Skilsaw (or equivalent), cut across the 2x4 at the proper distance from
the end, then use the Neander approach -- wide wood chisel and a hammer
to 'split' in from the end.

*WAY* less dust than the router will throw up.

Of course, you've got to be able to chisel a flat surface. for the lap.

To reduce the effort of Neandering, do a -series- of parallel custs with
the Skilsaw, then break the 'fingers' off, and clean up with the chisel.