I have some trees that are just too heavy to mess with every time I want to turn them, so I use the swivel seat bases off of small fishing boats on my benches. There are lockable and free spinning types. If you want a lockable turntable base for trimming trees they are very sturdy (obviously) and somewhat weather proof, but some are stiffer to turn than others.
I think they would be good to bolt to the top of the hydraulic platforms many people use on this site for transporting trees. If you are observant when you buy them, some have a hole in the centre and some are solid. The hole is a handy feature because once the top is screwed in place and the tree is on top, the hole (once you drill it through the plywood top) will allow the excess water to drain off/through... I've rigged up a system where I have a 5/8' automotive heater hose off of the bottom of each turntable, and the run-off is captured for other non-important plants, or simply directed away from other trees below it. Note: The hole is not 5/8" through the swivel... it's more like 2".
The lockable type:
https://www.sportsmansguide.com/pro...oat-seat-12-way-locking-swivel-base?a=1174769
The non-lock able type just don't have the locking handle.
What I consider the less desirable sold type:
https://www.seaflo-uk.com/marine-bo...ng-turntable-75301-fishing-cruiser-5385-p.asp