Superior threads sells a thread holder that is a bit pricey but can be changed to hold a horizontal or a vertical thread and also so you can use a bobbin as the top thread. It is made for a domestic sit down machine. I do not know how much space your longarm has nor if there would be room to attach this to the support for your machine. There is a neat video on the site that shows the difference between cross wound and stack thread spools and cones.