just leave a open channel.
Angle iron to rust and a square hole; no way. No wonder you can't get the straw out plus looks a lot of work to install.
If not using ready made as in posts above then cut it out of 4" sewer pipe, but don't necessarly cut it the full depth ( depending on what you need). Then just bed it in the concrete.
Set motorway crash barrier into the ground and concrete up to them leaving the dip in the middle as your open drain channel . Robust and not bad to drive over too . Won’t ever bung the grate up on themHas anyone since made any drains that don’t block with straw? I’m about to start doing drains in my calf shed but can’t decide on best way. Anyone filled drains with pea gravel?