So do I take it the difficulty of the Apollo 13 filter adaptor was more than getting a square peg to fit in a round hole...that it also had to direct the air properly for the filter to work well?
Indeed. Luckily the ECS had fittings for suit hoses, since it could be used to scrub air in the suit circuit.
It's not hard to find pictures of the CM filter. It's just a square box with the front and back square faces open to pass air. Because of how the suit fittings had to be valved through the ECS to rig this up, the crew had to figure out how to suck air through the filter, into a suit hose, and then into the ECS. That is, the hose worked by suction in this arrangement.
Once you see what you need to do, this is actually easily within the realm of the home MacGuyver. You really don't need much engineering knowledge, just practical know-how. And of course, the ability to use only what you have on hand. You can leave one face of the CM filter open and draw cabin air in through it. The trick is to adapt the other face of the filter to a hose that's going to suck air. This was done using one of the many plastic bags used to pack equipment for the flight. You just tape it around the perimeter to seal it to the edge.
Then you make a hole in the bag just big enough for the hose and tape the hose into it. But when you turn on the LM fans (which are prodigious), they'll suck the plastic up against the outlet face of the filter. That's no good because then only the portion of the filter right in front of the hose gets used. So you modify your adapter to allow a rigid structural piece that makes a "tent" out of the bag and holds it away from the face. For this they used the thick paper cover of the flight plan.
Starting over, you tape one edge of the paper to one edge of the filter, and then the other edge of the paper to the opposite edge of the filter. This makes a little Quonset hut over the downstream (outlet) side of the filter. Then you tape the plastic bag around it to make the airseal. You attach the hose to the bag as before, although obviously along the ends of the dome created by the paper.
You need a physical filter. That is, what we've been calling a "filter" all along is the lithium hydroxide canister that removes carbon dioxide. It doesn't really function as a particulate air filter, and you need one of those somewhere between the inlet and the fans because the fans and other parts of the ECS fit with very narrow tolerances. Particles that make it through the canister can foul the fans, and that's dangerous since the ECS is not crew-serviceable. So you wad up a sock (preferably clean) and slip it into the hose.
That's pretty much it. You turn on the circulating fan and it sucks air through the LiOH canister into the tented bag, into the hose through the sock, and into the rest of the system that removes odors, adds fresh oxygen, and (were the electrical systems operating normally) heats or cools the cabin air to a tolerable temperature.
When one canister is saturated, you just tape a fresh one onto the front of the assembly you already have. It doesn't matter that air gets sucked through saturated canisters; you just need a fresh one somewhere along the line and a means of making sure all the cabin air is forced to pass through it.