You could try duplicating a Broome restrainer. Picture here (or google Broome restrainer): http://www.fishersci.com/ecomm/servlet/fsproductdetail_10652_1475427_29104_-1_0 The Broome's sold by scientific supply houses are also clear acrylic, but if all you need access to is the tail veins PVC pipe should work. It is cheap, and easy to cut & drill. The squared end is just to keep it from rolling, which could be accomplished by glueing or clamping the finished restrainer to a small rectangle of wood. Please note that there are air holes in the nose end, which may not be obvious in the pictures. If you are only dealing with one size of mouse, the design can be simplified by trading the sliding plug for a simple cap on the end.