I rough cut the tail and made sure it fit snugly in the slot.
I then roughly shaped the upper and lower fins with the 1/4 inch poplar.
I chiseled out two notches for the fins.
I then inserted the upper and lower fins in place.
So far, so good. I will work on the pectoral fins next. These will need a little more effort. Once these are in, I can roughly sand the fins to 'thin' them out, and then work on sanding the body.