Securing Plastic container to Acrylic

We were looking for ways on how to secure our transparent plastic container holding the micro-controller onto the surface vessel,

We started playing around with L brakets to secure our container to a flat acrylic surface.

We felt that this method was not feasible as there will be a gap between the L bracket and the acrylic board (The L bracket was not completely flat on the acrylic board.

We then tried to research for other ways.