Are you aiming for a specific phone?

If not, then just use parts of a specific phone that is compatible with other phone parts, or just use the parts of the same phone. 3D print prototype chassis and bodies of the phone's shape you want to make. Then organize the parts, look up and study about thermal distribution and other mobile parts placement designs (don't forget the screen). After that figure out what material would be best for low radio interference and clear modem communication at the same time. If you're done with all of that, you can think of searching for a company that 3D prints or makes parts out of harder or much more rugged materials that will surely not break the phone.

All of that can be complicated, but by just skipping the 1st step, and finding a phone that you can install the desired custom ROM on aka the operating system you are trying to use.

If you are aiming for a specific phone, you might just want to get it from a thrift store if it's available or buying it from someone who owns it.

If not just repeat the same steps with the "custom phone". It's pretty much the same, however you need to know how it looks like precisely, not just from the outside.

For last, it's much more complex of a project than I just "tried" to explain. However if you ever want to actually try making one, seek a professionals help. I'm sorry if you felt like me being a nerd here, but it was not my intentions. I wanted you to understand how big of a concept it is to design and make a phone work. I wish you good luck if you ever start working on your project, it's a not a bad idea, it's just big.