The cheapest way is to attach rails to the existing handguard http://ant-supplies.co.uk/Pages/HandguardRails.aspx
Buy two - one for underneath to take the foregrip and one for the top to take the torch. They'll need the deft use of a drill to make holes for the screws but it'll be the single cheapest way unless you can find a second hand Magpul style one (assuming you have the right kind of Delta Ring).