Bite the bullet and get yourself a longer enlarging lens. Should be cheap enough on eBay these days. Go ahead and go for 100mm if you ever plan on doing 6x9.
BTW, moving the easel should not really cause this kind of vignetting. Sure, you could overrun the image and get the edges of the negative carrier blocking a side of the print, but that would be a straight edge; corners being clipped looks a lot more like inadequate lens coverage to me.