I'm cutting the slots for the T-Tracks in three steps. This is necessary for two reasons: One, I'm using a 1/2" cutter and my T-Track is 3/4" wide. The second reason, taking multiple cuts is much more forgiving and is much safer. Speaking of multiple cuts, I set the plunge router to cut about half the depth for the first pass and then reset it the final one. I'll show you more in a minute.

As you can see I've clamped the square tubing to my bench top. However, I'm not able to cut the slot all the way to the ends because of where my clamps are placed. That means I can only cut to within about five inches of the ends (like you see below), then I'll have to move the clamps and cut the remainder after I get this slot to the 3/4" width first.
 

What you see below are two different size shims that I used to end up at the 3/4" dimension. The slot that you saw above was set with the 1/8" shim to start with. I placed this between the router base and the aluminum block and tightened both #12 screws on the base. This arrangement places the slot near the middle.
 

Here is what it looks like after my third pass. And my width is exactly what I was after, as the T-track fits great and it sits just below the surface too. At this point you should have a better idea of how I went about all this. I did this process at four locations, which was right over the 2 X 4 stringers on the base (four places). That was the easy part, because now I want my last slot to be down the middle but I ran into a snag while doing that.
 

My last slot will be in the middle of the table (short side). And to cut the slot to within five inches of the edge was the same as before. However, for those slots I had the luxury of repositioning my clamps a small amount and then do the last five inches with no problems. That's not the case here because I'm in the middle of the table now and there's no edge to move my clamps to.

What I came up with was to install two pieces of T-Track temporarily (arrow). That way I could use a clamp designed to work with my T-Track to hold my square tubing like you see below. With these clamps in place I was able to cut both ends easily. And once I was finished with these last two cuts, I then removed the temporary T-Track. Whatever works!
 

Powertec also sells intersections. These came with screws and are cut with a 45 degree angle on them. This makes them very easy to install.
 

After all the intersection pieces were in place, I was ready to cut all the remaining pieces. And to do that I cut each piece to length on my miter saw, which worked great. Note the work stop on the left side, this helped keep each piece the same length. Safety tip: use a fine tooth blade here and clamping it down while cutting is highly recommended.