Very easy to do using a hand held circular saw and guide. Or table saw if you have one.

First drill the large holes at either end using hole cutter. Align guides and slowly plunge into material, saw will at first try to jump slightly until at depth, after that then just slowly feed.
You'll be surprised how easy it cuts.! I cut 20mm plate single pass on my table saw but with hand held saw I'd do it in 2 pass's.

Like Web goblin says rough out slightly under size, say 1mm, then clean up with router. I'd make a template and use bearing guided cutter to make this easier.
I made my first gantry sides using this MDF template and router method and while very scary at first it did a very good job. Took ages has I used very shallow depth cuts but it worked fine but I was cutting mostly full slot into plate and not just cleaning up edges like you would be doing with this method.

Jig saw would work but won't give has nice clean straight cut like circular saw does and take much longer.
Don't be afraid of using circular saw on Ali it works fine and I rough cut to size all my plate on table saw with a blade that is so knackered it struggles to cut hard wood but yet is fine with Ali.?? . . .Don't make sense but it works and has cut hundreds of meters of Ali plate. Thin curf helps.

You MUST wear FULL FACE protection not just eye protection and cover arms etc has the chips are red hot and will burn you.