Lightbox popup with delay on all pages? [Alternative to plugin: OptinPop]

I’m looking for a plugin that will pop up a lightbox window with custom content in it after a specific time/delay (Seconds, minutes, hours). In this particular case, I will prompt users to sign up for a free account before they can continue reading content. A user will not be able to close it or hide it until they become a registered user.

While the concept for the plugin OptinPop looked perfect, it just had too many bugs to even attempt to work with it.

Any suggestions from people who have initiated a similar feature?

Thanks!