For many days I was like:
HOW 2 DO DIS?

(Inlet blends with fuselage)
Then I had an idea. Here's how:
1. Build the size of the open face of the inlet first to plan the inlet size, using a fuselage.
2. Scale the inlet. Slim Inlets are about 1×0.5×1 blocks. (Which are those in the picture). Normal Inlets are 1×1×2
- first value is width
- second is height
- third is length
- for default rotation.
We must scale first and second values to fit planned size, and third value to something flat like 0.001.
Note: do not use 0. This will cause inlet inside (black) color to render improperly.
3. Place and nudge to proper position (thanks to HellFireKoder for fine tuner mod)
4. Profit
Note: I recommend all xml modding to be done as a subassembly as it is easier to find in case something goes wrong

Thanks for reading :D