What size cutters are you planning to use in alu? 50mm face on plastic wont generate much force.

Welding is you are all probably aware is an art in its self and you must only weld small sections at a time and move around the frame to reduce heat build up. Its slow but the only way to go.

You can design to minimise joints and increase contact area so spot welding can be used.

Is your model in AutoCAD? If so I could help out with the frame design. Hopefully increase strength and reduce the weight! :0)