Looking at your photo and trying to remember WAY back, did you mix sand in with the plaster? The distance from the outermost part of the fin to the edge of the plaster looks a little close. I think you will need a much higher sprue than what you will get from the height of that mold. Lastly, I went to a reuse center and bought an old oven for my work. Best of luck and I'll try to remember to check in more frequently.