cant see why you couldnt straighten it, at 2.5 m arches the roll is the correct angle,
staked out in the sun for awhile over summer and pretrty sure it would straighten out.
(farmers usually just attach it and slight curves but essentially straight)
add a few ties/tent pegs and i think you would be fine- here is reseller in SA
<http://blshipway.com.au/__files/f/3338/BLS_-_2014_AIR_SEEDER_CATALOGUE.pdf> - page 13cheers Boof63