Anyone here who has managed to build a small laptop with a Raspberry Pi? I would love to use my iPad via USB connection as a screen for it but not finding the right apps for my needs

Screen via USB... I never got that to work satisfactory.
I would try remote desktop over network.
I never tried it so no guarantee that it works.
Good luck.

Screen over USB is certainly possible. I'm almost sure there's a laptop chassis out there for the Pi, but it's probably based on the Pi touchscreen which is only about 7 inches in size.

@drRoss you mean the so called pi-top I assume. But I'm not willing to pay that much for a chassis, which i can print on my own :p The hardware inside is, what I need. And to make things even cheaper, I want to not buy an LCD but use my iPad