Not trying to knock anyone's methods, but this is a tried and true method that almost all car forums use to DIY repair worn mounts.
You can pick up the Polyurethane from any local hardware store like Lowes, etc for around $4-6. One tube is sufficient to do all the mounts. (You'll need a caulking gun as well, if you don't already have one - additional $2-3)
If you want to make it more aesthetically pleasing, then you can certainly use washers to make the PU look nicer once it sets, otherwise you can simply tape up one end and fill it. (Once it sets you can use a razor blade to cut off the excess and/or make it look nicer/smoother) Additional cost of washers would probably be less than $10.
I apologize for the poor quality of these pics, as this was done during a clutch job, so no actual DIY was planned for repairing mounts. Thus I do not have "finished" pic of the final result:
We simply taped off one end and applied the PU. We used a razor blade after it set to cut off the excess PU and make it look nice and flat.
HTH!
More NSX DIY's here:
http://www.c-speedracing.com/howto.php
Someone should try that with my end plates...Flexane is stupid expensive! I tried Silicone rubber and it cracked out in about a year, that is what lead me to the urethane! How does the polyurethane hold up over time?