I went with the gap I did, because it provides space for c clamps to be dropped through. I use 4-6" clamps and don't have any issues with them. It also allows 1" square stock for fixtures (with a little wiggle room). I chose not to have the slots fit the stock tightly because it eliminates any compensation ability if there is any angular error between slats.

At some point I'll be making t-nuts specifically for the gap, but I haven't gotten there yet. I intended to do that with my horizontal mill, but that has yet to be so much as plugged in (due to a lack of transformer; which I now have).

Something to keep in mind, you can't put a c-clamp on the edge from above, as there's nothing for it to grab on aside from the flange of the channel. So those need to be clamped from below on mine. It's not a problem though, since you can easily reach the middle of the table from the edges. I also have always intended on making the strap clamps so it's something I've lived with for the past year. If this was an issue, you could always weld angle to the sides of the channel to create a shelf for the clamp to purchase.