The roller is 12" well casing,the 3" is the tongue, and cross piece that bolts to the hubs. The hubs came off a trailer axle that got hooked on a rock in the middle of the axle so I knew the hubs were ok. The axle was 11/2" ,I think, square solid stock. The axle had brakes on it so I removed the drums and backing plate which left me with the part that was welded to the axle for the backing plate to bolt to so I used it for my straight edge. Blew a square hole on the end plate and welded the backing plate bracket to the end plate to be welded to the casing. I was kinda careful to square up the end plate with the roller when I welded it. I think it would have been better to use a self centering bearing on the ends but I didn't have one big enough to fit some 1"round stock I had. I cut a hole in one end of the end plates and welded a 1/12" coupling in it with a plug to fill it with water.
I know it will miss a few spots in the field but for what it cost me I think I can afford to go over the field again.
After we built I we saw a factory made one just like it except that it had a 24" drum and was 12' long.
Ill try to get some pics of the hubs when I get back to the ranch.
Happy farming !