If you built this from a kit, and know what you are doing, then scratch build a small battery tray to go behind the servo compartment, and move it back and forth inside until you achieve the desired CG, then attach.
If this an ART or RTF, and you have no building experience, then this may be a little easier for your first time.......
I wound up using the supplied battery/receiver tray (laziness). Place the battery to the rear of the receiver on the supplied tray, as the battery to the front made mine way too nose heavy. If you have a CG machine (these are relatively cheap, and invaluable), place it on the desired balance point, and if it is nose heavy, place a couple of prather weights (self adhesive lead weights) on the rear horizontal stabilizers, up against the fuselage (do not attach yet!). Experiment with moving different quantities back and forth until you achieve the desired balance with as little weight as necessary. If it is tail heavy off the bat, then go ahead and move the battery to the forward position on the tray, but this will most likely not be the case with the Avistar.
You do not want to place the weights on the back wall of the servo compartment, as this would require 3 times as much weight to have the same effect (fulcrum effect - the farther back you move your weights, the less you will need). Once you have determined how much weight is required ( I think I needed 1 1/4 oz.), go under the horizontal stabilzer(s) and, using a new blade razor knife, trim enough of the monokote from the fuselage to attach the desired weight. You don't want to attach any weight directly to the monokote, because if the covering comes off, so will your counterweight. I used a SMALL dab of thinned epoxy around the weights to seal any exposed wood, so the oil blowing back from the muffler would not soften the wood from underneath the weights, destroying the adhesion, thus having your weights possibly falling off mid-flight.

Hope this helps. There are many ways to do these kind of things, but this is my quick and simple way. I know it adds a little extra weight to the plane, but if this is your first trainer, you're not going to notice the difference like a 125mph pylon racer would.