How to diagonally connect something on onshape?

I need to diagonally connect a c channel to another c channel on Onshape, but it only seems to go horizontally and vertically if I use the fastened mate. Is there another mate I should be using or…? Please help if you know how to use Onshape or CAD! Thank you!!

I don’t really know what you’re talking about because I’m tired and don’t use CAD very often/am not good at it but maybe this is what you’re looking for?
image 45 Degree Gusset