Testing acoustic equipment before sea experiments is a necessary step, which usually requires large and expensive facilities. In this paper, we present the design guidelines, structure and details of a small-scale, low-cost acoustic chamber for in-lab testing of underwater acoustic networks. The chamber has been assembled with the objective to be of low cost and limited size: therefore, its installation fits small university laboratories that cannot afford large testing pools. The chamber was designed to mitigate the extreme multipath which, in a small chamber, makes communications unreliable. Considering this challenge, our chamber includes a phono-absorbing coating on the walls and floor, to be optionally complemented by a panel of the same coating material, to be installed at the water surface level. After providing the details of several phono-absorbing materials to motivate our specific choice, we carry out a number of transmission experiment with EvoLogics modems, proving that our design substantially reduces the severe multipath and thereby improves the communications quality.