There are several ways to mix mortars.
Very popular, but too expensive and bulky for small jobs. Storage is often impractical. Often cost effective to buy one for a job then resell afterwards.
Always rinse out before the cement sets if you want the machine to last well.
Can also be used to break glass into cullet. Add the glass and add a hard brick. Ditto scrap tiles.
See Paddle mixer
When used with haired or fibre reinforced mortar mixes these tend to form 'dead mice' on the paddles (clumps of hair).
Hand held paddle mixers for use in drills put a lot of load on a drill. A high power low speed mixing drill should be used. Using paddles in a cordless drill is pushing it.
Board & Shovel
It's not practical to use a plastic sheet instead of the board, the rigidity is needed.
Bucket & Shovel
Don't fill too full, or you won't be able to mix. Over half full is hard to mix. Some tendency not to mix material at the bottom well, so the bottom of the mix needs extra attention.
Fast Bucket Method
For small amounts it's faster to toss the bucket than mix with a small shovel. The bucket needs to be a quarter full or less. Shake the bucket rapidly up & down in an oval () shaped motion, and the contents go round and mix very quickly. At 2 tosses a second, a batch can be done in 20 seconds once the water is added. Energetic work, but very quick.
Very small amounts can be mixed on a hand held mortar board with a trowel. The same ring technique is used as with a full size board.
For really large batches it is possible to make a giant mixer driven by a car engine. These can be scaled up versions of a cement mixer for cement mortars, or a giant version of a food processor for Papercrete and similar formulae.
Suited to large cement mixing jobs, Papercrete and similar mixtures. Making one is not a trivial exercise. Purchasing an industrial mixer is prohibitive.