well...yes, you need to cut the top off the barrel. I don't see a need to filter the water as it enters the barrel. There will, of course, be some crud that collects on the bottom, but a good cleaning of your eaves, solves most of that problem. If you still want to have a lid on your barrel, try to find some steel barrels that have the replaceable lids with the hoop clamp. You may need to double up on the rubber washers to account for the rigidity of the barrel. Failing that, you could simply make a plywood lid for your plastic barrel that is slightly larger than the barrle itself so it simply sits on top...a couple screws in strategic places will stop the lid from moving side to side or blowing it off.
The nipples that I have installed are about a quarter to a third from the bottom of the barrel. From time to time as the barrel is nearly empty anyway, I simply tip it over and rinse it out. I do this once or twice a summer.
No, I don't have a link to these nipples. I assumed that they were all pretty much the same. A plumbing store or hardware store should have the basic pieces necessary to fit something together if the nipples are incomplete...it isn't a complicated operation with special sizing or anything?
Good luck.