My first advice is to ensure you've read the gantry design threads:
I started with the advice from these threads but somehow moved away from the L-shaped gantry. There is no reason to not connect the two horizontal parts...

But if you make the plywood mock-up first ( with badly drilled counterbores that will be adequate if not neat) you will then have a fully functioning CNC router to do the job for you. Yay!!
Oh! This is a very good point!

I think I will reconsider my plans as I went today to one of the local university's metal shops and there there is a Deckel FP2 mill that I could use to flatten the surfaces, so I can avoid both aluminium and epoxy! Does it make sense to first fully weld the bed and then flatten the x-axis surfaces? I don't know how to make sure that both sides are in the same plane, but maybe this is not necessary?