This is what I do. I use it on my Carvin AC50 for an upright imitation. I just use ordinary polyurethane foam. It's fairly soft so it doesn't push strings sharp. I just shove it under the strings at the bridge. polyurethane tends to self-destruct after a couple of years so you have to replace it ever so often but no biggie.
You control damping by both the distance from the bridge and foam width and thickness. Low strings need more damping than high strings so you end up with a sort of long pyramid of foam. You can adjust damping by how far you shove it under the strings and where you place it. Also you can adjust high to low damping by angling the foam to give more damping to high strings or low. Works quite nicely.
Only downside is compared to mechanical dampers that you can flip on on off with a lever action the foam takes more time to install and remove.
