Campervan light weight ply such as Morland board is 15mm thick. Single lip and double lip t-trim is 18mm wide. The 2mm slot should be central. I keep a sample of board with the slot cut in my router box to check slot cutter depth is correct each time I use it, as I dont have a dedicated slot cutting router like most pro convertors do
Obviously the supplier here may be using different board/t-trim.
Obviously the supplier here may be using different board/t-trim.