About this detail of the Tiger
The breech of a gun, like the Tiger's 88, has an important role to play. It contains the explosion that drives the shell forwards. Therefore it is a solid block of steel.
This photo is from a Late Tiger in Saumur museum. The green paint is not original, but we can clearly see the breech block at the left of the photo. Notice that the opening in the rear is machined through solid steel. On the right side is a large lever.
A close-up photo shows how the breech is sealed. A steel block, called the Breech Block, is held in the center of the breech. By turning the lever, the loader can lower this block, exposing the hole in the core of the breech. He can then insert a shell and raise the block behind it. You can see the rounded top surface of the block, at the lower left of this photo.
The Hachette kit doesn't provide the solid interior of the breech, nor the breech block. You can see the assembled kit parts in photos on this blog.
You could improve the Hachette Tiger by constructing the interior of the breech, using e.g. plastic card.