I regularly use flexible rubber lines ( 5/16 inch is the correct size, I think) for trans coolers without a problem. It will slip over the existing line, just be sure to use 2 worm-drive hose clamps at each end.
Go to a decent auto parts store and buy some in bulk. The best stuff is the reinforced rubber lines intended for the low pressure side of a power steering system, or the line used for auxiliary transmission coolers.
Alternately, you can use reinforced rubber fuel line.
I should mention that OEM-style steel cooler lines are also available at a decent auto parts store. You should be able to get straight pieces in various lengths with the correct flares and fittings. I think 5/16 inch line with old-style SAE fittings (NOT metric) will fit.
Just measure the length you need and bend it to fit (use the old line as a guide). The line is pretty malliable and is designed to be bent a lot to get the right shape.