a ring seat 57 thus is formed and flange 65 is located so that canisters 25 sit snugly on ring seat 57 within flange 65 resisting inadvertent separation by virtue of such fit flange 65 extends partially along sidewalls 75 of adjacent canisters 25 at the opposing ends of such canisters as such horizontal boundary 48 is not as readily subject to separating forces from blows received at such boundary since such blows strike the outer wall of flange 65 0034 fig8 shows still another device for reducing inadvertent separation of opposing surfaces 50 of adjacent canisters 25 that is a tie down strap 69 in the cavity defined by inner wall 45 fig9 of cover 21 strap 69 is fastenable relative to one or more canisters 25 to exert inward compressive force on the canisters to resist separation for the opposing surfaces thereof in the illustrated example strap 69 extends from the bottom of the cavity inside cover 21 up opposing longitudinal sides of cover 21 and extends across the inside of the cavity at the top of cover 21 the strap thus encircles all canisters of the set received in cover 21 and when cinched exerts the required compressive force to hold the set of canisters 25 together although the preceding embodiments have shown drum shaped canisters substantially filling volume 41 of cover 21 alternate shapes of canisters are within the scope of the current invention two examples of which are shown in fig9 and 10 in fig9.