Some of the responses have indicated that I failed to give more detail.
Hopefully, this will help.
I want to build a grid of interlocking pieces of plywood (3/4"). Each long piece in the grid will be 4 feet long, the cross piece is also 4 feet. The height of all the pieces is 4 inches. On top of this, I'll put a sheet of 3/4 ply. I don't need to make the top super flat as in a torsion box just ok will do. The idea is that I'll be able knock this down when I need to, which I expect could happen over time. Want to end up with a grid 4 feet by 4 feet.
I was thinking of a grid of three pieces in length and 5 cross pieces with one at each end.
The interlocking slots need to be all the same depth, width and in the same spot on each corresponding pieces (each cross piece needs to be identical, each long piece the same).
I was thinking of a dado cut, but just not sure if an 8" dado would cut that high, and to be sure of consistently in the slots, I'd would have to gang the pieces together. Seems to be a bit of over engineering.
I was thinking of making a template out of MDF or ply. A router could be used to ride the template and cut out the slots.
Someone suggested to me use of a jigsaw, but I really want super straight cuts and each to be the same. Not sure if the jigsaw is the right tool for this job.
I looked at the Kerfmaker - nice tool, but am barely able to afford to buy MDF these days then another tool. Besides, it looks like it's better for a tablesaw, then a router.
Hope that helps.
MJ