ok, first attempt using sketchup 8

[Jonathan]

It's only a suggestion, take it or leave it. I post these things to benefit other people who may want more from the machine than Dean.
Adding the plate to make a box is very simple and should make a big difference but if Dean never wants to cut aluminium regularly there's no point. 0.01mm tool deflection might not sound like much, but when you start cutting metals it is - surface finish, material removal rate and tool life are affected. Running the calculation with cutting force for aluminium gives 0.058mm.

I used 8mm web thickness for the RSJ, not 6mm, in all the calculations as that's a standard size - with 6mm tool deflection is about twice as much.