Use a diamond wheel tile cutter? Something like this Plasplugs model
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Pack up the 'table' to give you the right depth of cut so that it doesn't cut both sides of the U, and use the guides to keep it straight. Might wreck the blade eventually, but ally is soft so it might be recoverable, and they're cheap enough to replace anyway.
A table saw with a TCT blade will do this easily enough (wear ear defenders and safety glasses!) or you could make a simple wooden jig and either use a router or biscuit jointer.
I would expect aluminium to clog a diamond wheel very quickly.
We used to cut a lot of aluminium for one product we made. To do that we used a wide toothed saw blade with flood lubrication. Aluminium needs lots of lubricant, both to cool it and to stop it grabbing the saw.
FWIW the Evolution saw blade that is rated for (thinner) steel and aluminium as well as wood will happily cut a few mm of aluminium without any lubricant. I don't know whether this is because they have a slightly different geometry to a standard TCT tipped wood blade, or because they just have smart marketing.
Anyway, on the strength of this I have both a "portable" and a sliding compound mitre from Evolution, which both serve my needs very well. (That said, I don't do fancy carpentry).
For your case, I would use the "portable" saw and probably make up a suitable jig with bits of timber or ply, on a plywood base of appropriate length, so that you could clamp the ally to the jig which would provide both a flat surface to support the saw, and a guide to run the left hand side of the saw baseplate against, rather than messing around with a fence attached to the saw.
My understanding is that a clogged diamond saw blade can be cleaned up by running it against a carborundum block, in this case presumably on both sides of the blade. Is this not the case?
No idea, I've never needed to find out. However, even if it did work, you would probably need to spend more time cleaning than cutting. Aluminium is really clingy stuff.
It's possible. But aluminium is not hard, like say concrete or stone You shouldn't use diamond on it - TC is ideal with a decent coarse tooth profile saw.
Not aluminium. but thin, fragile and very, very, *very* clingy: corrugated plastic roofing. I had to cut some sheets to length last week: I tried half a dozen of my tools over about an hour, and ended up with a raggy-ended sheet.
So of course (duh!) I consulted Mr Google. One chap there used a multitool, using the fine segment saw blade. I'd hardly ever used my multitool in all the 2-3 years since I bought it (a bargain at Aldi, not to be passed over): I was very sceptical: applying a heavy, vibrating, mechanical tool to an awkwardly shaped, fragile material?
But my gosh - what a revelation! Not only did it work, I cut my first length in 3 minutes. A perfect clean cut, with no damage to the sheet.
So anyway: aluminium is not plastic corrugated roofing - but worth a trial.
Although the vibration seems rapid, the amplitude is small, so the sliding speed is kept down. As a rule of thumb, there's a limit for the product of speed and contact pressure if you want to avoid melting.
Variable speed jigsaws are also good for this, once you find the right speed. But no good for the OP's job.
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