Well, I tired of trying to to grok menuentry via /etc/grub.d/40_custom…But, as I write, 20.04 is finishing up its install on my ASUS notebook 
I had transferred the ISO over my local network to the notebook while still pursuing the menuentry path. After I gave up, I decided to use an older 2.0 usb stick instead of the 3.0 usb sticks; I put the 2.0 into the usb port on the notebook and used the “Startup Disc Creator” from the Ubuntu menu on my desktop to make a bootable usb drive. After the process was finished, without ever removing the usb drive, I rebooted the machine and set the notebook to boot from the usb drive. Which it did! The older ASUS notebook had no problem with the 2.0 usb drive as a boot drive, despite rejecting the 3.0 usb’s as boot drives. I guess the old dog didn’t want to learn new tricks!