Butter Keepers really aren't hard to make. After you catch on to how they fit together and how to measure the top and bottom parts in such a way to guarantee they will fit well, you can easily throw them with confidence.
A good butter keeper should have a large flat area on the top so that when it is opened and the top, which is filled with butter, is lifted out of the base, the top will sit flat on the table and not wobble when the knife is inserted into the soft butter.
I make a measure tool similar to a Japanese tombo (http://www.bambootools.com/tombo.htm). I use it at the wheel to measure the inside volumes for the water-bath bottom half and the top lid in order for them to fit together neatly.
Take two skewers (they must be pointed at one end) and tie them together using a twist tab. You can snip off the blunt ends to make them shorter and easier to use. This photo is a close-up of the knot. It must be loose enough to allow the sticks to slide up and down and from side to side easily, but also stay put. The pointed ends give a precise placement at the center of the bottom and the edge of the inside rim.
Sometimes when I get a size I particularly like, I will take a waterproof pen and mark a line on each stick to keep that measurement just in case it might slip out of adjustment.
Or you could have a master set of skewers and just glue them together to keep them rigid. A set of different sizes could be used as a "master" set and compared periodically with a "working" set.
(I'm showing how to measure the base and top with a finished butterkeeper, but you would do this with raw clay.)
Throw the base first. This will establish the size of the butter keeper. Keep in mind your clay shrinkage and that the lid, although smaller, should hold at least one link of butter compressed.
With the base still on the wheel, line up the sticks so that the vertical is perfectly centered with the center point at the inside bottom, the horizontal piece level. Adjust the horizontal stick so that the point is lined up with the inner edge of the inside rim.
Keep that measure. This will tell you the inner volume of the area that will hold the water-seal part of the butter keeper. The butter chamber in the lid must slip easily inside and not rest on the bottom or floor of the base. You need to have space for the water seal to cover the butter and keep the air out.
When you throw the top, you can eyeball the clearances by holding the skewer in the same way and guage how the measurement of the top is smaller in height and check that the width is narrower than the bottom. If you wish, you can make an "inside" tombo to use.
Another check of how well the two parts will fit together is to invert the top while it is still on the wheel and hold it over the base and look to see if it appears to fit.
Also, don't forget to allow for wiring off the top and bottom and be sure to make those parts just a tad thicker and you want the finished product.
Sidenote: How do you get skewer sticks to stand up in a butter keeper base? Muffin to the rescue!
The next two diagrams are from a hand-out I give every person who buys a butter keeper.