The classic industrial approach with massive porcelain jars (which by their very nature makes sealing the materials inside difficult) is really taking a sledgehammer to crack a nut, as is often the case with old (tried-and-tested" traditional designs - who in their right mind would use a cone wheel when you can have thyristor-controlled DC motors for full torque at 1rpm?
I set out to design my ball mill using basic materials, minimal woodworking skills, and minimal reliance on outside help. Essentially it is just a rotating box (cage) with a lid, inside which is snugly fitted a standard 5-gallon plastic drum. The drum should be about one third filled with flint pebbles between 1" and 1.5" diameter. This will hold 20lbs of dry material with 2 gallons of water. The drum is filled with dry material via a funnel, and the whole thing can be lifted in and out of the cage easily. When milling is finished simply unscrew the lid and pour through a 40s mesh sieve - the pebbles remain inside the drum.
Unless you can weld the bits you will need to get made are two 6"x6" plates with a shaft welded to the centre point of each and dead true. The shafts should be the same diameter as the pulley holes. I found that with a standard 5-gallon drum with a diameter of about 10" I would need two 2.5" and two 12" pulleys; this steps the motor speed down to around 60rpm for the drum which is about right.
The whole machine can be made from an 8'x4' sheet of (ideally) 1" ply - I used shuttering grade rather than best WBP - and several feet of 3"x2" softwood. The first part to make is the cage - which should take the 5-gallon drum as exactly as possible - too loose and it will thump about in use, too tight and it will be difficult to get the drum in and out. I use a recycled screw fitting to keep the lid closed, but any quick-release device would do. The two 6"x6" plate/shafts are bolted to each end ensuring that they are as true as possible ie in line.
The rest is so simple I feel the picture tells you everything you need to know - it's really just a case of making measurements to suit the parts you are working with and cutting appropriately. I would recommend the belts which are made up of multiple segments for coarse adjustment.
The most extreme use I have made of this machine is a 72-hour session with porcelain clay for David Winkley - apart from the background noise, no problems. I always mill all my glazes for about an hour, mainly to get a really thorough mix rather than reduce particle size substantially.