I also have Roundcube installed on my server.
For my personal use, I opened my mail page and copied the link then created a desktop shortcut to it.
When I clicked on it, I had a login applet that I used my email address and password to input.
From there, the option is to save the password or not (FireFox).
This bypasses the cpanel login.
I know this is not exactly a customised login page, but I hope it helps.