Hi James, Luke.

Regards the gantry and the way you intend to use the angle, like jonathan I feel this is not enough and will be weak area.
From your rough drawing and the fact you mention using Ali 20mm plate have you thought about using 20mm for the gantry sides and bearing plate.
Doing it this way would let you bolt the bottom extrusion to the bearing plate which the ends would be bolted into the gantry side also the 12mm front plate will be bolted to the gantry side plate all could be located into shallow machined groves for easy setup.
It would save buying the 180x45 which really wont had any extra strength and would be very strong.

I've done a few quick drawings see what you think.! . . . It's could use a bit more bracing etc and could be fancyed up a bit but it should give you the idea of what I mean.


Cheers