I need a dash speaker

[BigSix]

So I bought the Retro Sound radio and a speaker. The speaker came from one of the online suppliers to our trucks (can't remember which). It has two small speakers in it and is mounted to a nice black plastic. Problem is there is no way it would fit my truck - there is nowhere under there to bolt it up! Anyway, I think a single speaker will be fine. I can always get a more powerful single speaker - I'm not picky on sound - plus the diesel won't be something I could drown out with any number of speakers.

So what single speaker can I run under there? Is there one that bolts up?