I know this is from a while ago but I'll add an alternative for the next person doing this....
First I wouldn't use a circular saw with a concrete blade, if this was outside it would be fine, but not inside. The reason is the silica dust, that stuff is proven to be toxic and people can die from it. .... Plus that concrete dust gets everywhere and I mean everywhere with a dry saw.
When cutting in new footers for new posts in my basement shop, I was strapped for cash, but I already had a grinder and circ set up for concrete. So I used scrap to build a "glove box" that looks sort of like some sandblaster compartments.
It was just 4 scrap plywood walls with an old salvaged fridge shelf of safety glass for a top (still wear safety glasses). No bottom piece. Glued on 3/4" weatherstripping around the bottom edge of the walls to seal against the floor. Magnet mounted lights inside.
Walls had five holes....
1 cord for saw
1 shopvac hose (vac outside)
1 garden hose (water + elecricity means.... plug saw into GFCI)
2 holes with sloppy fit for my arms; slop served as air intake (with a nice cooling effect, rushing into the cuff of my leather gloves)
Had to cut in small nibbles then move the glove box, but I wasn't doing tons and wasn't on the clock.
I also kept myself on a small plywood platform up out of the wetness on the floor.
Used cheap pool noodles to make a small dam and settling pond on the way to the floor drain. This really worked well, the water reaching the drain was running clear.
I was nervous only about the grinder's wheel shattering or electrocuting myself, but it all went smoothly. I'm something of a dustmask fanatic, and this setup worked so well I didn't bother and did not regret it later.
The disadvantage is time and monkey business, getting it all set up and then cutting just a few inches before having to move everything for the next few inches.