If you have WinUAE, then just create the file in Windows - this is for Windows 10, not sure whether it works in earlier versions or not as I've not got anything less to test on:
Open a command prompt: Press the Start button, then type cmd
Code:
cd /d %UserProfile%\Desktop
fsutil file createnew test.adf 901120
You will now have a blank ADF file called test.adf on your desktop. Mount it in WinUAE and then format it with the file system you want.