A family member had 'old school' masons build a significant amount of stone walls on his property. They mixed the mortar right next to the sand pile. A Case 580 scoop of sand, some bags of type 2, mix well, and scoop it up with the machine to get it to where it is needed.
One guy was mixing something like 8-10 yards of mortar this way, in addition to moving some stone as well.
The plywood idea sound a lot neater, wish I used that when I was setting granite slabs last year and belgian block jobs.
I used a mixer for belgian block once and I thought it made the mix too wet.