Before you even dare to try this, get this:
https://gametechmods.com/forums/index.php/topic,6845.0.htmlfirst. Already got it? continue reading.
1. Make a rough shape off what you wanna build, then after the first component(which is placed flush to the wall), for the next component afterwards, place it right next to the component before it.

Count the amount of pixels the component needed to hit the wall or other components, and remember it, cause you will need it for the next step.
2. Go to the chassis design tab, and then on the board, shrink the chassis down the opposite way the component moves on step 1(move upward-shrink chassis downward, move downward-shrink chassis upward, NOTE: if the component moves either left or right on step 1 (on a box shaped chassis)divide the pixel needed by half and then shrink both left and right of chassis the half of the pixel needed) and then, adjust chassis height the same as before, pick armor, and then redo the entire component placement once again.

3.Rinse and repeat step 1&2 until either the chassis is airtight, or until you hit the minimum chassis shape limit.)


Final note: i'm sorry if you guys already know this technique before.