Here is a really great idea I've used many times on all types of instruments.
Simply using a bunch of common houshold rubber bands. You can buy a whole box for a couple of bucks. By stretching them just a lttle between the bar support posts or eyelets you can make sure the bar never touches the timber underneath. If your marimba or vibraphone bars clunk a little when stuck hard because they are not quite high enough off the timber, then this will completely fix the froblem without compromising the sound AT ALL. The photos below show this idea used on the P1 instrument. The photos show the rubber bands on top of the orange tubing but in fact the tubing is not needed at all if the rubber bands are used. You would still need something around the screws to isolate them from the bars but the tubing used for this is not so critical.
You can see the basic technique below. Each rubber band is twisted once betwen each peg so the contact surface on the bars is smaller and a little thicker. Small bands can be used betwen two pegs, or if you have longer ones you can stretch them between many pegs. As long as there is a twist between each peg and just a little tension on the rubber bands. Here I have used two bands betwen each peg for a lttle extra buffer.