The picture of the plug cut down before finishing appears normal. Usually bubbles are caused by the plug curing too fast for the air bubbles to rise out of the mixture. Try mixing the plug slower and/or coloring on the darker side so they aren't as apparent. You should also be filing the plug with a bevel knife after cutting it down. If these don't help try switching brands of plug. I've always had good luck with Brunswick's plug (better for lower temperatures) and Ebonite's Power House Everclear plug (be careful as this can get too hot too quick if mixed really aggressively.)
Hope this helps