Am I asking for trouble with this? (manual milling?)

[Web Goblin]

I would doubt that your B&D router would be up to the job to start with. The only way i could see you doing this would be to rough it out first with maybe a jig saw then clean up the edges with the router but it aint going to be easy and to be honest I dont think you would get any where near a decent finish.
Try posting an RFQ on here and see if you get any offers.

[JAZZCNC]

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.

[GEOFFREY]

We used to cut sheet steel using a tipped blade in circular (10" skillsaw I seem to remember) and had no problems. we did however use a wax/gease stick to lubricate the blade and the cutting line on the material. Jazz's comments about safety are really important. G.