If my suspicions are correct you are known as Moorcroftman and the grass box you are having problems with are for a Ransomes Ajax!
Please be aware metal sheeting which this box is made from can age harden, so where possible heat should be applied first, this can be with a simple blow torch or if your workshop is better equipped a oxy-acetylene kit with a rosebud torch. When warmed you can use the round end of a ball pain hammer, and place the box on anything hard, as long as its the same shape as your finished box. OR use a panel beaters maul behind. Hitting it can stretch the metal so be careful, if a crack appears, its best to braze the area, as welding could do more damage if heavy handed by burning a hole. If however you find it too difficult, then look for a company that does ducting work, ask them nicely or offer them a drink!
After writing this I could do with a drink as well!!!
Kind regards
Andrew
.
