If you have it moving and have a new t-stat you are almost there. I think the pieces in the picture are right. The t-stat does need a place to seal against and the piece below it is beveled to accept the roundish shape of its bottom. Since you are now experienced at getting this apart and back together, you need to try some adjustments.
As I was saying before the relationship between the airflow and the mixture control is such that when you get the airflow right you will have this area of mixture control within tolerances most likely. Let me point out that when the airflow is right it will still suck your finger in but it will not drop engine rpm by more than 50. I personally like to get it to nothing as these systems are often rich for more than one reason. Also beware that almost evry time I fix one of these so simple problems, someone has already messed with all the other adjustments (which absolutely should not be messed with till the temp mixture correction has been corrected.
With all that said lets shut off the airflow. You have noticed that to finish closing off the airslide you need a longer t-stat. Well make it longer. Add small washers to the area where the end of the t-stat sits. Start small as we do not want to go to far. Remember that when its all done you are going to have to open the airscrew as it will not idle high enough when the add air is properly shut off as you realized. The most important point with this additional air/cold enrichment adjustment is getting the warm setting right as this is where the car is mostly driven and is almost always the cause of way rich mixtures.
Bosch Master, ASE Master, L1
33 years MB technician