There are several ways of doing this, there is no one method fits all as everyone's tastes are different.
I'm going with some 3M window weld. It has what I believe to be the right amount of hardness for the results I want. A little more rigid than stock, but not jarring the fillings out of your teeth.
I tried another part with some Vulkem 116 but that stuff comes out gritty and really stinks. The 3M windo-weld is much smoother, sets up quicker and doesn't smell nearly as bad.
Windo-weld has a shore hardness of around 50. If I recall correctly a couple other companies like prothane and energy suspension who make inserts that's around 80 or so maybe even higher. In reading other reviews on it I find the following theme. “I got used to the vibrations” or “this vibrates way too much.” That to me reads too much vibration for my preference. But that's just me.
For the one part polyurethane like this I think 50-60 is about as hard as you get. It is very easy to dispense though. For harder poly fills you'll need to go with a two part pourable polyurethane. I have heard of some people using stuff called steel weld, maybe that's like jb weld?
So on with a couple pics, it's very very basic so don't expect much.
Masked off one side.
If you are so inclined you could probably take some time to smooth it out with soapy water and a glove. I'm not too worried about that.
This stuff does set up decently fast, it's a chemical reaction with the atmosphere. The warmer it is the quicker it will set. I tried another part in the oven it seemed to help, though at the cost of many irritated looks from the lady friend.
As some of these bushings are a bit thick I'd say give it a week for the. To fully cure.
I've also heard of some people putting an air hole through the bottom part of the polyurethane. Not sure if that is necessary, if so I may revisit that at a later date.