Just to be clear, that isn't a flask, or a container.
This is the hood mechanism that goes over the flask. The triangular piece at one end is the fixed / latch end, the rest of the hood slides away so that the actual flask (that sits within - and below!) can be removed. There is a good picture here https://www.railwaygazette.com/freight/nuclear-decommissioning-authority-orders-flask-wagons/41734.article showing the hood slid right back and the flask being craned out from within.
Your model railway your rules of course, but it's not an equivalent representation of how the things work.