Forget welding, too thin. Glue would be best, look up aircraft construction. To be light enough to be a top you would mainly have a thin sheet top and the braces carry the load. Make a wooden top and load it down at the bridge plate and measure the deflection, do the same for the aluminum and adjust till you get the same deflection. Would it be ideal? Probably not but it gives you a starting place.
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Fred
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