If water gets trapped in the welds then yeah it'll rust pretty quick. It's really a question of minimizing risk instead of eliminating it. Like, weld the front vertical edging so that water/muck can't get inside, and leave the rear vertical edge so that whatever moisture gets inside can flow out. Similarly, make longer welds on top so water can't get in as easily, and smaller welds on the bottom so that water can escape easily. Another option would be to just use bolts on the bottom, which might be adequate when combined with welding the top, depending on usage. Primer everything before and after and coat it all with underbody spray and you'll reduce the risk even more. If you're still freaked then best not to use a welded stiffener.