There are various ways of cutting the slot depending what tools you have. The way I do it is, drill an 8mm hole at each end, then very carefully cut the top and bottom using a super thin 1mm cutting disk on an angle grinder, punch the middle out with a hammer and punch, clean up the slot with an 8mm rotary burr in a die grinder and touch it up with some black paint.
If you have minimum tools available, I would drill an 8mm hole at each end, then a series of small holes along the top and bottom as close to each other as possible, then some more 8mm holes along the centre, then punch it out and clean it up with a file. The infill is reasonably thick so the small holes have to be close to weaken it enough to punch out.
Cheers, Murray

