Kevin, if a person is boxing a section of a C channel, they are welding a plate to the 3/16 edge of the channel. Most of the plate is in midair. The elliptical shape/fish mouth is in midair. It will do nothing to alleviate a perceived stress riser.
Montana, I see you are holding a bracket in your hand. I assume this is what we are trying to mount in this post. I will assume you are holding it where you want it to be located. If so, make a rectangular plate from scrap steel, stitch weld it to the C channel and bolt the bracket to it. Or use a piece of rectangular tubing as a spacer and bolt it all together. or use a C channel the same size and bolt one leg on top of frame top leg and one leg on top of frame bottom leg. (two "C"s bolted together). You are not boxing a frame, you are trying to attach a bracket. Think outside the "box". Modify the bracket to be bolted maybe. Bolts are strong and permanent and this stress riser talk will end.
Can you give us the part number and company of the kit? Many of these kits are bolt on. May be your question should have been to ask how to mount that bracket. It should not have a been a boxing question. We may have some good ideas for mounting the bracket if we can see what was supposed to be done and what mistake was made. Glad to help.
My opinion: Try to avoid welding to a frame. I think you are thinking what I'm thinking. You are forming a plan. That's good a thing.
