Rivnuts are a great piece of kit but dont over do it with the ammount in structural areas as they need quite a large hole to fit them in
I thought m6 isnt that big when i first used them but if i remember correctly they need a 10mm hole to insert them into, which is quite large on a 25mm rail if you whack loads in every few inches
A good alternative is the self tappers that they used for fixing metal cladding to buildings, you can get them in various sizes and they have a hexagonal head on and their own cutting tip at the end
You get the bit to suit the head in your drill and they drill their own perfectly sized pilot hole and then screw into it
Some also come with a little crush washer that is water tight after tightening
So a 5mm self tapper bolt would only need a hole half the size of an m6 rivnut and are probably equally as strong
I think they sell them in screwfix if anyone has read my ramblings
