I have not done it but for your garage I think PVC pipe would work ok there. I think if you made it the length of the arch way, it could press against the side walls and be forced upwards to match the shape of outline. Attach that on each side to two parallel vertical sections (one behind the other) separated by 2 90 degree elbows, and a t-piece that could rotate to match the angle exactly. Glue together all parts except to allow the T to rotate, maybe a big cable tie to hold that together which could then be snipped and allow for easy storage of the whole lot. The Smaller one, maybe some ceiling hanger wire bent to match the shape?