Walling off lava lakes

[Guswut]

I see yours are a bit smaller than the max 21x21.

Two 9x9 farms (18) with a three block space in the middle (21) puts them at minorly smaller than the max size. I decided to keep the look of four 9x9 farms over the 21x21 farm, although it'd be fairly easy to switch. But that's an old world, and one that I don't care to deal with any longer.

Although I do have Ender Buckets now, so it wouldn't be as horrible to place all of those water source blocks (or just the bunch needed at the top, which is what I ended up doing in the end).

 

[Hydra]

I just placed them in 'stripes'. 1 21 long row of cane, one 21 long channel with water (only 2 source blocks), 2 rows of cane, one water, 2 cane, etc.). I did work out the most effective setup (quite a bit more growing spots) but actually setting it up took longer than simply creating a double layer farm so I could not be bothered anymore.

 

[Guswut]

I just placed them in 'stripes'. 1 21 long row of cane, one 21 long channel with water (only 2 source blocks), 2 rows of cane, one water, 2 cane, etc.). I did work out the most effective setup (quite a bit more growing spots) but actually setting it up took longer than simply creating a double layer farm so I could not be bothered anymore.

Using the optimal pattern (which is fairly easy to figure out), you only need to set up one 21x21 area once (at the highest Y point over your farm). From there, after you've set that up (use scaffolding to get up that high, or pillar, or whatnot), you can make as many 21x21 sized farms as you'd like. Just fill in the areas around the waterfalls, and you're done.

It's a whole lot easier than making more floors manually, especially if you enslave a turtle to do the work of placing the dirt blocks (and cane, too, if you really want to make it easy).