Let me know when you plan to do one, I'd like to watch and see how it is set up. I've not done done and am not quite sure about the lift mechanism with the signs and water. Also, there seems to be some discrepancy about whether you need to be within a certain number of blocks from the spawner or not within that distance. Only one answer can be right - do mobs spawn within about 16 blocks of the player or not within that distance?
Sorry I didn't see this earlier. I've done the basics for the skele grinder just need to make it look nicer and more accessible. I'll show it to you when we meet online.
When you go a certain distance from the spawner and if it's still visible you can see it stops spinning at that point you know it's not going to spawn any monsters. I think it's the 16 block rule, but I usually just guess it I know it works when monsters start dropping down the drop shaft.
If you want to be sure, just dig a straight line from the spawner until you see it stop spinning and shooting out flames.
This is of course just to keep the monsters spawning, they can go a long way after they spawn, so as long as your within 16 blocks and they end up close to you, your good.