No need to block off those areas that remain on-screen while you farm. You will have to block off areas just off-screen, but that holds true even if you did keep those spear traps.
I did initially mirror what was on the left side so that I had enemies coming from the left and right, but that was before I realized that limiting the spawns to one side only didn't affect the overall spawn rate. If you have spawns on both sides the ravens won't be as efficient in helping kill Mourning Woods since their attention will be split between the two sides. Plus, you'd need even more spear traps.
I recently installed a teleporter directly above the slime chamber. This allows for easier loot collection after the Moon event is over. There is a switch directly above where I'm standing, connected to the actuated teleporter between the two campfires.
Some loot will be lost because there is a 400 item limit in the world, but the lava does help in burning away some of the drops of small value (the spooky wood, coins, and hearts). I have obsidian platforms above me that have one purpose: to trap the Headless Horsemen when they teleport in from the spawn area on the far left (the inbound teleporters are in the lava directly above my character). Without the platforms the Horsemen tend to run to the left once they teleport in, out of raven range. With the platforms the horsemen jump up onto the platforms, and get trapped where all the event bosses are, and the ravens hit every mob/boss there simultaneously.
However, I use the platforms to set final wave time records. The platforms are best omitted if you're planning to farm so that they don't trap the low value items, and instead let them fall into the lava.
You can expect around 10 platinum per Pumpkin Moon event. Not bad for an AFK farm
. You'll have to make a lot of trips back to home base to sell everything off though! As a result you might consider installing a King or Queen statue nearby to call a vendor NPC and sell off the items.