Welcome to Iceshanty and welcome to the valley. You say you've tried most of the easy to get to spots. Does that include Long lake on the Glenn highway (mile 86 I believe)? There are some burbot in there. You say upper Mat-Su. Have you tried Big lake and Byers lake? Both have burbot in them. Another option is the Big Su. I've caught burbot through the ice on the Big Su using set lines. A good spot to try is the mouth of Rabidauex Creek. Good luck.