The best way is to scratchbuild your own frame rails or use C channel from Plastruct or Evergreen of the appropriate size.
When I splice a kit frame, I include an insert of brass or rigid plastic super glued (or epoxy) inside the frame rail. I try to run the insert
the entire length of the splice/additional rail if possible.
When I shorten a frame rail, I will also use an insert to strengthen the splice.
I used rigid plastic for this dual spliced rail.
I fill in the visible seam and sand smooth, then add bolt detail.
Tim