I've used (and had used on me) a mini-blind wand several times, but its effect is more that of a cane than a paddle, plus you don't get the full benefit of it when used OTK.
As for overall size, it depends on your use. If the top is standing, as in a school paddling scene, then a longer paddle is better. If it's OTK, the longer paddle is difficult to wield and will be focused primarily on the further cheek, with the closer cheek getting much less impact. A shorter paddle minimizes that. The other consideration is a paddle meant only to cover one cheek at a time, much like a ping pong paddle. It allows the spanking to be more even on both sides.
I think 3-4 inches wide is a good size. Much wider than that and the edges won't impact the flesh, which makes for some real sore spots. That said, rounding off the corners is a good idea, but if you round the edges, you'll miss that extra feeling. On that subject, holes in the paddle help, too. But place the holes randomly; if you put them all in a line along the length of the grain, you'll weaken the paddle.
I would use oak, which is what the classic school paddles are made of, or maple. With cedar, I don't know about the strength issue, but it's softer and doesn't hurt as much. I'm not sure, but I think cedar may have some substance in it from which some people get an irritation. That's not part of a spanking.
I don't know about acrylic. I know some people have reported plastic paddles have shattered, which is a potential hazard. Lexan has a good reputation for paddles. It causes more sting than wood, but doesn't last as long and usually doesn't leave bruises, so it has its advocates. Again, holes in the paddle are good.
Building up the handle and including a loop, so the top has a good grip on it is helpful.
I prefer a well sanded, but unfinished paddle. Others prefer a nice finish.
And then there are so many thing around the house which can be "repurposed" for a spanking 😃.