Mix the plaster thicker, about cake batter or pancake batter thickness.
Note that my lego mold has a lego bottom. That also keeps the wall the correct thickness too.
Some will seep into the less than perfect blocks anyway. No biggie, just soak and clean in the dishwasher when is wifey gone. I put them in an onion sack to wash them.
This is one of my older plaster buildings I did from a mold I made from a DPM and latex. These walls are less than 1/4 inch thick and the middle window sashes are flat toothpicks put into the mold and cast into the plaster. These were bad about breaking until I started putting toothpicks into the trouble areas while the plaster was wet. The wood acted as rebar.
Fred