I should first say that I didn't invent or create this filter, I just tweaked it a little. That being said, this is sort of the final evolution of the old underbed gravel filtration system, it's just easily manageable and cleanable.
This filter works great for removing fines and unlike a bead filter has lots of bio action also. It's one drawback is it has "speed limit" of about 2000 to 2500 gph before the 22 1/2 inch diameter causes the sand layer to become a fluidized bed.
First you have to install the drain of your choice. I like two inch shower drains because the lip on top is very thin and won't let crud build up around the edge.
I use either the straight or the 45 deg shower drain depending on the room I have below the tank. Forget the gasket and just use sealer on both sides. The surface of a 55 gal drum isn't even or uniform enough to make the gasket useful and it will just squeeze out where it's the tightest.
Also wipe all sealer off that squeezes out. Sealant just takes up the space between what it's sealing and any excess will probably just peel loose over time anyway when submerged oin water. I like Polyurethane sealants but use the sealant of your choice.