Countersink and screw it in. I use flat bar for the sides of my box. Angle irons aren't square through the bend so when you lay them on you have to screw them in from the top anyways. If you screw them in from the sides it'll push the tops up as it tightens.
So I can counter sink a flat bar? I didn't know how easy it would be to screw through metal like that. I see what you mean, I'll definitely try that.
Yeah, countersinking metal is super easy. I have my screws sitting a 1/16 of an inch below the surface. No hang ups at all.
I'd recommend skipping the HDPE and getting UHMW Poly instead. HDPE gets really dry and slow. UHMW is a little more expensive but it's slick and durable. Mine's 1/4" thick. You'll have to round the edges and make sure your edges are completely detuned and rounded over because you'll def catch if they're square.
Another problem is that I can't find any flat metal strips that are wider than 2 in. If I use the angle Irons, all I want to know is whether they would slide or if I would catch. If not I think they are the best option for a 3 in. wide
Countersink with a drill bit and the screw will be flush if you do it right, just google how to countersink metal and I'm sure you will find something, super easy.
I thinkk trex would get chewed up pretty bad.
See if you can find 3/16" HDPE from a local plastics supplier. Might be cheaper than steel, definitely easier to work with and will slide better. Not sure about the corners of the rail though.
I'm buying a ton of HDPE, so I might have some left over for a flat rail. Not sure how I feel, It would kind of just be like sliding a small box, right? Also, I've seen a lot of people use PVC Pipes as the topping, how would 3 PVC pipes side by side work on a rail?