this is what i used, but only bought 1 metre. http://www.ebay.co.uk/itm/Steel-box-sectio...58874940b3
just ask them.
you can see it has rounded corners. it slides in snug. you then scribe you holes and use a pillar drill to get a good hole. being a good fit, means it can be slid out again, without a hammer, BUT you might want to add a small face to the metal to tap, should it become stuck.

it's strong! -you can stand on it!