I like this simple little thing. It sounds good, really good when you're using a slide. It's easy to install. It's only slightly obtrusive when Playing I use it as a forearm rest and I'm not sure if that impacts the reverb or not. For the price on Ebay this is a sweetie pie.
Price: $14.95 (new)
Where Obtained: EBay
This unit attaches under the strings on strings on your dreadnaught and rests against the top and side of the guitar. It installs in a few seconds and uninstalls just as fast. It really delivers on the reverb sound. It's hollow and has a couple of springs inside so I think it's really based on the old spring reverb tank. It's a black plastic jobbie the size of a couple of decks of cards. It only slightly gets it the way of my playing style.
It's seems a little flimsy but I haven't given it a major thrashing yet. The little metal pieces that hoo under the strings may cause your guitar to detune a little and maybe cause early wear out of the strings but I can't prove that yet