What about wisps? They arn't trapped b a conveyor belt.
Besides, The major problem for me is that my only big sized magic biome is far away from the rest of my base. Most to the time, I will be too far away from it to work.
Edit: I can't seem to find a mob router on the FTB wiki.
How do you get that much leather? I have a cowfarm, but it can't produce the multiple stacks needed for that many nets.
Wisps will be caught by a conveyor belt if they pass through the block it's in, which happens now and then (it happens to me, too, when I'm flying around inside with a jetpack, adjusting things). It's why wisps in a conveyor-based mob trap end up gathering in the drop zone, even if they never fly out.
The mob router is an MFR block which you pair with conveyors to send mobs of a particular type in a particular direction. As with the planter, and the item and enchantment routers, the colors on the block's outside match up to the colors in its GUI. Look up its recipe in NEI. By the way, if you set up a disenchanter, you can use the enchantment router to help auto-disenchant mob drops before you send them to their ultimate fate.
As for how you get lots of leather, you can auto-spawn cows (most of us have plenty of mob essence), or you can use a breeder and chronotyper to send your animals to the grinder (you'll want a good wheat farm). Grinders don't kill baby animals.