Consider using a cheap, small used 2.5" drive for your operating system, then using a USB 2.0 enclosure with a cheap big 3.5" drive. You can put some foam under the enclosure if you do a lot of off-roading or stuff that might damage a desktop hard drive.
Doesn't take up space in drive
Lets you use cheap hard drives
You might need to figure out a way to power the enclosure.