I cut slits in my 3/8 copper manifolds, one in the mash tun the other in the boil pot. After experimenting with a couple of cutting tools, like a hack saw with several different metal blades, an angle grinder with cut off wheel, a air powered die grinder with cut off wheel, and a dremel with cut off wheel. I settled on the thinest kerf of then all, a dremel with a thin cut off wheel that cut 1/16 slits. I cut the slits half way through the 3/8 tube at 3/8 spacing. It cut pretty quickly and was the easiest tool to control while cutting.
The manifold works great, drains fairly quickly if needed, and filters well without letting in any crushed grain. I did make one unexpected discovery though. On the mash tun the slits filter best and drain the most wort facing down. However, on the boil pot the slits drain and filter best facing up, especially if using pelletized hops, because the trub and fine ground hop particles settle to the bottom. Facing up, the slits avoid sucking most of it in. Whereas, facing down, the slits did a good job of sucking a great deal of trub and hops off of the bottom of the pot. With whole cone hops it's not such big issue, but the slits still work better facing up to avoid the trub. With slits cut half way throught the tube, facing up they only leave about 3/16 dead space on the bottom of the pot. IMO, that's a small sacrifice of wort to pay of a pretty clean wort in the fermenter.
One additional technique that works well to hold the manifold tubes, elbows, and T's together is to fasten them with a thin SS wire through tiny holes drilled across the fittings. That is, instead of soldering a copper manifold or glueing a CPVC one. Then the manifold can be easily disassembled for cleaning.
Also don't forget to clean out the cutting debris in the tube after either cutting slits or drilling holes. I used a shotgun bore brush to help strip out the ragged bits on the inside of the tube that weren't totally detached.