Usually after download, the installation will start. You can just stop the installation without any ill effect.
You can then create an USB installer with the downloaded Mojave app.
(1) Create the USB installer via Terminal with "createinstallmedia" (a minimum of 16GB UFD is needed or an external USB drive)
sudo /Applications/Install\ macOS\ Mojave.app/Contents/Resources/createinstallmedia --volume /Volumes/"target_USB" --applicationpath /Applications/Install\ macOS\ Mojave.app --nointeraction
Note: target_USB = name of the USB drive you're writing to. When done, USB Drive will automatically renamed to "Install MacOS Mojave"
(2) Install Clover with following config, add appropriate Bootpack to /EFI/Clover
UEFI:
Install for UEFI booting only
Install Clover in the ESP
Drivers64UEFI\DataHubDxe-64
Drivers64UEFI\FSInject-64
Drivers64UEFI\SMCHelper-64
Drivers64UEFI\ApfsDriverLoader-64
Drivers64UEFI\AptioMemoryFix-64
Drivers64UEFI\PartitionDxe-64
Install Clover Preference Pane
Finally add to Drivers64UEFI\HFSPlus.efi (not included in Clover installer)
Note2: The new format will be APFS, there's no way to use HFS+ unless you clone the drive to an HFS+ formatted drive.