In that case, have you checked if putting the bolt and nut in the hole, and tightening them up, does then "close the gap" sufficiently to allow the clamp to be inserted in the arm?
If that doesn't look like it will be successful, then it would appear that you have a poorly-manufactured clamp - at this point, you would have to make a decision, possibly one of the following:
- Take the issue up with the supplier to get a replacement - but that will most likely involve
sending the whole dish kit back to him and waiting for a replacement, which would could take a couple of weeks or more, depending on where he is located, AND how quickly he responds to the request (some suppliers are distinctly "better" than others in that area!)
- try modding the existing clamp as you previously suggested and/or possibly removing some plastic from between the arms of the clamp so that they will pull nearer together when the nut & bolt are tightened
- Buy a "standard" Triax replacement clamp kit from ebay, Amazon etc., for about £12-£13 delivered , although some kits are priced at £18- £19 (in both cases, ridiculous prices for a small plastic moulding - bit like one of those old Airfix kits - IMHO!)
- Buy the one that
@ozumo identified for about £20, and solve your problems, and "for good" it would seem.