A homemade wrist leverage bar can easily be created by adding weights to one end of a plate loaded dumbell and holding it from the other end. Use an olympic dumbell if a thick handle is desired.
Though plate loaded leverage bars are marketed as a state of the art wrist training tool, many find work with a real sledge hammer to be more motivating.
The jump in difficulty between plate sizes is very large, so progression in the plate curl is typically achieved by affixing additional small weights to the plate until the wrist gets strong enough to move up to the next level. A couple of pony clamps or c clamps work well for this. Moving the added weight further out on the edge of the plate will make the exercise harder.
Due to the stress placed on the wrists, it makes sense to gradually work up the weight on this exercise. Starting with a 10lb plate, make the first target 2 10lb plates clamped together. Try to progress to a 25lb plate for reps. Plate curling a 35lb plate is attainable, but will prove to be a very challenging goal for most.
Progression for the plate wrist curl is achieved using clamps to affix additional weights to the plate, similar as to in the plate curl. The big difference here is that due to the dynamic motion of the wrist, weights that are manageable on the plate curl will prove to be quite a challenge on the plate wrist curl.
Key factors to consider when selecting a wrist roller are the diameter, length, knurling, and how smoothly it will roll: