I have recently installed a double swivel seat base in my Ford Transit Mk8 2018 LWB (L3H3) from https://www.customshopdesigns.com/
Keeping two belted seats at the front for my conversion was crucial (3 children).
I had been waiting for stock since around October 2021, so I was excited when I was finally able to order it towards the end of February for £319!
I don't envy anyone manufacturing conversion bits for transits because there are many variants, so this is not a criticism of custom shop designs, but as expected from reading about others' experiences, installation didn't go 100% according to plan.
The _only_ problem I had was that the bolt that I was supposed to shorten (if required) to 40mm was already 35mm:
with the top 5mm without thread, so I couldn't fasten the base to the floor without modification:
Fortunately, I was able to remove the seat mount (or whatever it's called):
remove the bolt with an angle grinder, and attach another of the correct length (40mm).
Beyond this little snag, the process was pretty seamless.
It is a little tricky to operate, but given this is the only double swivel in town, and I absolutely must keep the double seat, I'm pretty happy.
Hopefully this helps others who might encounter the same problem or otherwise be worried about installing this swivel.