Having been in machining most of my life I suggest you get a 1/8" carbide double cut burr with a round end. They are moe expensive than steel but will last many times longer especially on abrasive material such as fiberglass. The round end allows you to drill straight through when needed and also helps in smoothing things out. If you can get some tap magic squirt some inside the spinner and it will keep the burr from loading up with the soft alluminum. Kerosene will also work to stop the loading but is more flamable. You can purchase the carbide burr at many hardware type stores or tool stores. Next to my dremmel drum sanders for wood this is the cutter I use the most.