I may have a (paper) picture somewhere of a case that I made for a wooden Cutty Sark model that a friend made. The model was over a metre long and nearly as high. I persuaded him that we could make it appear to 'float' in the case. I made a Perspex box, just higher than the draught of the boat, with a cut-out in the top that the boat 'sank' into, up (or down!) to its waterline, and a green marble Fablon covering to the base of the case. All this fitted just inside the main Perspex case and it did work very well.
Marking out the shape of the necessary hole was delicate, but fairly easy once the boat had been mounted on blocks at the right height, and a vertically mounted pencil, cut to the waterline height was moved round the hull to mark the shape of the hull at the waterline - this was then cut out of the horizontal part of the internal box.
I used Perspex for this case as it would have been seriously heavy with glass of a suitable thickness in it - and then realised that cutting a boat-shaped hole in it was quite feasible.