nice find on the poplar. I would try to find something else than oak. cherry,maple and walnut will make very fine calls/. there are a ton of different wood to use but you should not worry about than now. that's later.
whatever , you use for a table saw fence. be should that fence is clamped down some how, and yes, you need to watch your fingers,. to lay out your box, start at the bottom, find center of the block go 3/8 of a inch on each side of center, cut your 5/7 degree angle, upwards fro the bottom. the thickest part of the box is the top, narrower on the bottom, looking from the end of the box, not that hard to do. just take your time , watch your fingers at all times and have fun,,,,,,,,,,,,,,,, chris