I have a 2022 model year Plus Four and can tell you that the sidescreens are extremely thick and bulky compared to the Traditional cars. Pondered a sidescreen bag and reckoned I would struggle to lift it with it's contents and that it wouldn't fit behind the seats unless you had the seat as far forward as possible. After some trial and error came up with this solution: Take the passenger sidescreen and turn it so the flat top edge drops down the drivers side of the car, flat edge nearest the door, behind the seat. Reverse the procedure with the Drivers sidescreen. The Memsahib is a dab hand with a sewing machine and ran up a couple of individual covers in thin soft cloth to protect them. I should add at a fraction of the cost Morgan wanted for their bag! Once you have got the hang of it , very easy to do and makes a neat job of storage as well as hiding anything on the back shelf.
Not sure about "the additional room available internally compared with a Trad". I am sure if you got a tape measure out the cabin of our Plus Four would prove bigger than our previous Traditional Roadster but, there is far less useable space overall. No room for maps, no door pockets, the glove box is passable but the extra expense lockable panel behind the seats is tiny and not worth it. The space behind the seats is smaller than our previous Roadster. I think the major culprit is the thickness of the doors which eats into the cabin space in a big way. Having said all that it is far easier to get in and out of than a Trad.