Very little is known about the real Roman subarmalis, but it is clear both from a few cryptic references, and practical experience that something must be worn. A good and very durable subarmalis can be made from leather, with sheepskin used for padding on the shoulders. Some people have made them from linen, some from wool, all with equal success. You could also use layers of thick felt for padding, and in fact one of the few references is to felt, with a separate leather cover, for rain.
I used a 2-3oz soft handed vegetable tanned leather for mine, and built s second layer over the shoulder so I could insert the padding (sheep skin, four layers) in between. My goal was to keep the whole thing as streamlined as possible to aid in donning the Newstead by myself (which is more like putting on a steel pullover vest than a jacket). It is a simple vest pattern, with an over lap in front and three thong ties. The pteruges were first built, then glued and stitched to a leather "belt", which was then attached to the vest. I had planned to let it age naturally, but had the brilliant idea of using chemical gun blueing on the inside of my lorica. This turned out to not be so brilliant when the vast majority of the blueing transferred to the leather, giving me a girdle of black stripes. Forth with the dye... Here are some subarmalis links: LEG VI VIC select Equipment, then Body Armor. LEG XX, scroll down to find Subarmalis. LEG IX HIS go to Gear, then scroll to Subarmalis. LEG VIII AUG MGV go to reconstructions, then Trajanic shoulder.