I would have thought a frame builder / car repair man who uses brazing, would be able to braze the broken bits together again, then sleeve both sides, with a wrap around the centre, and the add a couple of gussets to strengthen as well. Looking at the design, it looks as it an additional cross brace, across, near the top of the lower section, would strengthen it up considerably. When you get it fixed, I assume the upper bars slide in to the lower bars. If so cut a length of narrow inner tube, then roll it onto the lower part of the upper bars, insert in to lower bars, then roll down some of the inner tube , to make a waterproof sleeve.
If not , what about Kevin at D-Tek, he may even be able to repair it for you, but try a local frame builder, / hobbyist first.