Just use FK8 then you only have to have one side machined; on the free end just make up your own bearing block and use a bearing with 1/2" ID. On the drive end you'll have a bearing journal at 8mm and the drive journal at 1/4". Mount the flange bearing block to your top plate. Really, you don't even need a bearing at the free end since the z axis is so short.
I believe Gerry and a few others "machined" their ACME screw ends by putting a cutoff wheel in the table saw and making a jig to roll the screw with the blade raised at different increments. Another way would be to make or buy a reducer coupling. 1/2" on one side and 8mm on the other; then get some 8mm drill rod, cut a length that's long enough to go through the bearing block and still have room to attach your motor coupling. I've done it this way on a few builds and it works just fine. Mill or file flats if you use set screws.