I decided to have a go at one of these too. I have a universal arrangement that can go in any boat, which relies on an extending plank and a seat on a clamp that can be removed to go directly on to a thwart...
However, the simple short mount is easier, if you know for sure you are going to be able to sit it on a suitable thwart...
It's also less likely to damage the thwart.
I have a trip coming up where I am going to be 3 in a boat, with me in the middle or bow. The plank is a non-starter, so I could just remove the seat and clamp and take that. However, I also have a spare seat and swivel plate from the days of having 2 set-up options. So, I went and did a wee job on it, and very nearly painted myself into a corner...
I took the swivel plate off.
I cut 20 inches off a suitable plank of wood.
I marked the positions of the holes and drilled the wood.
I counter-sunk the underside to take the heads of the bolts.
I bolted the swivel plate to the wood.
I cut a piece of anti-slip rubber sheet and upholstered it to the underside of the wood. (My upholstery skills will not worry anyone on BBC's 'The Repair Shop'

)
Then I went to mount the seat back on the swivel plate and realised I should have done that before mounting the plate on the wood! Aargh! It was all very awkward trying to get the screws through the holes in the plate and into the seat with the wooden base attached. However, it wasn't easy to remove the plate from the wood, as the heads of the bolts were now hidden under the anti-slip mat!!! With reflection, I probably could have removed the nuts and put it back together without needing to access the heads of the bolts, as they were tightly buried and the nuts tightened without needing to put a screwdriver into the slots of the heads. However, I just went for it and fiddled about until I got the nuts through the holes in the plate and into the seat. I felt like Ed China trying to get the bolts back into the alternator of a 1972 Aston Martin.
Got it assembled, anyway...
Watching the folk I know who have this arrangement, it seems much easier to use a pair of Velcro straps, rather than bother with ratchet straps. So, I have ordered a pair of 1m Velcro straps.
So, anyway, if the OP hasn't done it yet, after all the drilling and everything, I would put the plate back on the seat first, and mount the plate to the wood second.
Col