Dayton Audio Exciters (also called audio transducers) are namely used for applications where you don't want the actual speakers to be visible or in plain sight. You can purchase these here:
There's a product video on there that demonstrates a guy using them on his jet ski.
As you may have already guessed, the performance of audio transducers can vary significantly, depending on what type of surface they're vibrating. You will really want to test out different surfaces with the transducer before jumping in and using acrylic as I have. These transducers are meant to be permanently affixed to whatever surface you apply them on, so do some testing and choose wisely.
Personally, I've tried using them on my desk (wooden), but you have to crank up the volume on the amplifier significantly (because of the sheer mass it has to vibrate). I've also put them on my wall, and at max volume, you can hear it on the other side pretty clearly. Foam project boards also work pretty well too.
The main reason I chose acrylic is because of looks, but the choice is yours.
The best way to test these is to use double sided tape on top of the adhesive protectors, that way when you remove it, you'll still have the adhesive protector on the transducers.