I would use an air bladder inside of a woven “sock” (they are available in carbon, kevlar, and even fiberglass). You insert the bladder (deflated) into the dry “sock”, then slide the entire assembly into the extrusion. Once in the extrusion, air is applied to the bladder inflating it (and the “sock”) to the interior dimensions of the extrusion. At which point, an epoxy is “infused” (or drawn through with vacuum) into the fabric. Once the epoxy is cured, the bladder is deflated, and removed. The part can then be removed.
Note: Prepping the extrusion with mold release prior to the start of this process should aid in removal the finished part.