I think they do still sell just the complete lower portion (base) your concerned about and they have the throttle blades already installed. They did a year ago.

I use thick ones like them but without the flap for the carb to base seal and they work well.

In one of my setups I get lucky enough to use 9 gaskets, 6 thin between all the aftermarket plates and the 3 thick ones for the carb to top plate for the good seal and to help keep heat from building up in the carbs.