Here's an improved version of the sequential adapter for cheap chinese H-pattern shifters. It fits tightly enough on the shaft to not need any screws or clamps, and is much more less likely to rotate than the one which ships with these shifters.
I made 3 different versions with different throw lengths, I personally prefer the middle option.
For the versions which use HAL-effect sensors, you need to install some magnets in the pockets. I personally found using 2x 5mm diameter, 1mm thick neodymium magnets stacked in each pocket works very well, glued in place with hot glue

NOTE: let the hotglue cool down a bit before inserting the magnets, as high temperature can demagnetize them!

Note: It takes some force to lock the shaft on its place. I recommend using atleast 4 walls and 50% infill. Generic PLA works just fine.