I hear you about the cost. 4 rails at $10 each, add Tee bolts and handles at $10, and it starts to add up to real money after while. I don't even want to think about my costs. Add software and whew.
Slots are important for ease of assembling your cutting project. In my case, I used the extra foot of Tee rail as a clamp. I drilled two holes for the tee bolt. I was wishing those holes were slots. Look at my clamping picture for clarity.
I've heard of some guys building their t-rails from wood. That might work too.
Regarding your grid, do it, but you will need slots in clamp bar for sure.