I tried the cheap fix which consisted of wrapping aluminum tape around the
boxes. This didn't last too long (4 days/2,000 miles). A better fix is to
take them to a welder, who will charge you perhaps between $10 and $40
depending on how hard it is to reach into the corners and how hard it is
to build up on the rust. Afterwards paint the tin with POR-15
paint-over-rust paint. In my case, it was worth it, because even junk yard
quality, badly rusted and perforated "type 4" boxes go for $150 a pair
here in the rust belt. 

Sami