The minimum thickness I can print with any level of structural integrity is around .020" (scale), which equates to 0.2" real life. Those air cleaner walls are 0.035" scale, which would equal 0.35" real life. Could probably go thinner there, that would be a pretty heavy duty cleaner in real life at more than 1/4" thick
I tend to print the first examples with a safety margin to see how they print, then tweak them down to a more accurate dimension. That air cleaner base came out first try, the lid took about five redesigns to get it to print properly.
There is also assembly to consider, there has to be enough surface area to glue one part to the other. Although ... I guess I could try to find a tiny stud for the carb and mount the air cleaner real life style. But I'm sure the scale stud would end up being like 1" diameter in real life size ... and pretty sure you can't find 1mm or like screw size 2 wing nuts
And yes I'm playing with the idea of drawing up an air filter to go in there. I'm still learning this scale model thing ... I had numerous failed lids before it occurred to me I could make the bottom of it flat since it won't show anyway. The printer really didn't like the hollow space under the lid.