Unfortunately my Harbor Freight trim router died after two uses so I wasn't able to fully realize the full plunge-router-style replacement base for it however these parts were functional in the short term.
First is a replacement for the stock plastic base. Apart from the geared screw it should work as well as stock. I only used it once so no comment on durability.
The second is a 'plunge box.' The stock (and replacement) bases should slide vertically in this base allowing you to start routes mid-board fairly easily. The small screw holes are for small circle cuts.
Not sure who would find these useful but have at it...