The ABS endcaps are thought by some to be structural. It is believed that they don't have any ribs supporting them because they act as the ribs themselves. I'm not a structural engineer and can't really comment on this, but thought you should know, before pulling them out.
To get the look you want, I think you have a couple of choices. The first is to fabricate an entire metal endcap on a bench, and then install it by riveting it along the edges, like your current ABS endcap. The idea here is that the bench-fabricated endcap unit should be at least as strong as the ABS unit, so if it IS structural, you won't lose any strength. This has been done before, AirForums member Zeppelenium discusses it here starting with post #64:
An alternative would be to leave the ABS endcap in place, and just build an aluminum facade over it. This, too, has been done in different ways by different people. Frank aka Overlander62 covers his method in his blog, here:
Anna Lumanum: here's where I'm at...