Use whatever size medicine bottle you want for the size of your blocks. Mr. Granny actually found the square bottle after John had already made the round blocker, so as soon as I find the material to use for the inner disk, I'll make myself a square blocker. All the parts, other than the disk itself, will be interchangeable.
Cut the bottle top off at the dotted lines. Mine was cut at the label line, which makes it a bit too deep, so I don't fill mine completely with soil. You need something like a real sharp box cutter, or maybe a Dremel tool to make this cut. I did make one with an electric miter saw once, but I shattered a couple of bottles before I got one right, and my hand came dangerously close to the blade.
Drill a hole in the bottom of the bottle, just slightly larger than the bolt you will use. You don't want the bolt to fit too tightly, it has to work as a plunger and move easily through the hole.
Here are all the parts you need. (1) a drawer knob with a 3" bolt (2) the bottle (3) a nut (4) a disk made of a solid material...this one is plexiglass, cut to fit loosely inside the bottle, with a hole drilled in the center (OK, almost center LOL) to accommodate the bolt (5) an acorn nut.
Insert the drawer knob bolt into the bottle top.
Screw the nut on just far enough to leave room for the disk and acorn nut. If your bottle isn't too deep (mine is) this can easily be adjusted after assembly.
Place the disk on the bolt.
Screw the acorn nut onto the end and make sure everything is tightened. The acorn nut will make the seed depression in your soil block.
Prepare your soiless mix. I prefer to use a premade potting mix (NOT potting soil). I sift it through a kitchen colander to remove large particles, and add some vermiculite. I don't measure, I just add until it feels right. Don't get too much vermiculite in it, or the blocks won't hold together.
Wet the mixture with warm water. You don't want it too soupy....think of the texture of cooked oatmeal. It should hold together when squeezed in your hand.
I scoop the mixture into the block maker, and press it down with my thumbs. You can see that mine isn't filled. If it was just a quarter of an inch shorter, I would fill it completely. I press the mixture in very firmly, you don't want loose blocks!
Slowly depress the plunger to expel the soil block. If it balks, just give the handle a slight turn and the block will come right out.
And here is the finished product. A perfect little soil block with a depression in the center for the seed. If it sounds like a lot of work to make them one at a time, it really isn't. I timed myself, and it took me 13 seconds to make one block.
Add your seeds and cover them with a bit of vermiculite. Cover them with a humidity dome and keep them in a warm place. You can water them by misting with a spray bottle, or bottom water by carefully pouring tepid water around the base. When bottom watering, I try to give them just enough that the blocks absorb all the way to the tops. If there is still water in the bottom of the container at that point, I suck it back out with a turkey baster. It doesn't take many tries to be able to judge the right amount to give them. These blocks were placed on a plastic tray from the dollar store, then covered with a clear plastic salad mix container. The plastic clamshell containers from the bakery or deli work great, they have their own built in humidity domes. Of course, you can use a regular seed starting flat with humidity dome.
Now...if you've done everything right the blocks will be quite sturdy. You should be able to pick them up and move them around if need be. Then, when you're all through.....go clean your fingernails!