The commercially available trays and blocks for making dubbing brushes are easy to duplicate and can be made better. Dubbing brushes are great for making large streamer bodies.
My tray makes dubbing brushes that are 23" (58 cm) long, and are less wasteful of material.
The board is 3/4" oak, and the groove is 1/2" (12 mm) wide X 1/4" (6 mm) deep, the top edges are rounded off. A cup hook and dowel are in the cordless drill to spin the wire. The cordless drill just lies on its side at the end of the dubbing tray. A nice coat of varnish seals the wood and helps the dubbed material spin on the slicker surface.
A removable peg on each end of the groove aligns the wire for spinning. Align the edge of the peg with the center of the groove. Rubber gasket material and washers form the wire hold downs at each end
Wire for brushes can range from 34 to 30 gage (AWG). Magnet wire can be bought is bulk from electronics stores or beading wire from craft stores is cheaper. I re-spool the wire onto thread spools for use in the ceramic tip bobbins. Instructions on how to make dubbing brushes and tie flies with them are available on YouTube, search for "dubbing brushes". The only trick on this style of tray is starting the spin since the table does not drop away from the wires for spinning.
Once the setup with wire and material is complete, grasp the wires on the far end next to the peg while holding tension. Start the drill spinning and the brush will start to form at the drill end. Once going, raise the wires at the far end to allow the twist to continue down the length of the dubbing brush. Once you do a couple, you will understand the technique.