#22 check valve should go in that hole. On my 2003 boxster it would slip out no barbs or ridge to hold it in.
I got a 3/16 brass barbed fitting from the hardware store and some 4mm silicon tubing.
I put the barbed fitting in the hole, and connected the tubing to the other end of the fitting and then the check valve.
I used another barbed brass fitting to plug into the "Y" connector.
Unfortunately the brass fitting into the manifold is hidden here behind a hose. I will try to get a better photo next time I am in there.
I also replaced my vacuum reservoir as it was leaking.
When done I had 18"Hg on a vacuum gauge I installed.
After a month of messing around my car cleared all monitors in 200 miles and passed CA smog!!