Yes, a 0.5x focal reducer will cut your focal length from 650mm (I think that is the focal length of the 130 slt) down to 325mm and the whole moon should fit in your field of view. Actually, a 0.8x reducer would probably do it if the diagonal of this sensor is indeed 5mm. But it would just barely fit. I prefer a bit of space around my subjects so I don't think I would have gone with the 0.8x reducer anyway.
The downside to the 0.5x reducer is they typically are not the best quality toward the edges of the frame but that might not matter much with the moon in the center of the frame.
Personally, I would just stitch together four images to get the full disk of the moon in the field of view. That would give the best results. The 0.5x reducer should work though.