For your needs I'll second the Unimates. They're old-fashioned dummy couplers, have to be lifted and hand coupled, but once together they stay together, and will manually couple with MTs just fine if you ever need to. I have them on all of my F units, rear of As and both ends of the Bs, and they work fine.
Feather River Trains has 4-pair packages of all three lengths for $3.00, to give some idea of the price.