I have now purchased a used, but like new, Lexus 19" wheel tyre size 235/40. This is slightly larger than the 225 size which Lexus fit as this seems to be unavailable or hard to get.
The wheel fits easily in the boot and only sticks up by 3" above the floor level.
The next step is to construct a new carpeted boot floor with a raised area similar to Big Rat's post.
I will report again in due course.
In the meantime I wonder if any member is aware of the bolt size and thread for the centre fixing bolt?