Can I get a Windows 10 trial on a USB drive and use this to install on a custom build?
You can buy windows 10 on usb or you can install using the windows media creation tool.
If you have a usb flash drive, over 4 gigabytes, preferably usb 2, for drivers purposes, google windows 10 downloaded, go to the one on microsofts website, then, while the tool is downloading, google rufus, and download that, when rufus is done downloading, open it and select your flash drive and plays it to fat 32 and ntfs, stock settings, then when the flash is done, download windows onto the flash drive. Then when windows is done downloading, plug it into the pc you just built and boot to the flash drive, then download windows with whatever version you have. After windows is done downloading, go to cortana and search recovery drive, select the dive you used to download windows and make that into a recovery usb, it'll come in handy in the future, and be sure to label the flash drive so no one else deletes its contents. Just so you know, you don't need an activation key as soon as you download windows, just some of the features will be deactivated until you input a valid key, which can be entered under updates and you'll see the sub folder to input the key in. Happy Building!!