I wonder if making a clamp to close on the top of the mobile goal is legal or not for a descoring design, or if there are any limitations I should be aware of.
Yes, there are some limitations, all of which are detailed in the game manual.
Take a read, then come back if you have any other more specific questions. Remember that if there are no rules preventing a strategy, it is probably legal.
When reading rule G7 which is the one in question I would be careful to note the difference between a field element which the ladder is (however read the exception) and a game element (which the mobile goal is. Just to be clear rings are scoring elements so they definitely don’t fall under G7.
In G7, would that mean if I closed the top part of a wall stake, then it would be illegal, but legal for a mobile goal?
Your interpretation is correct. The wall stakes is listed as a field element, so clamping onto it would be a violation of G7.
This is what happens when we read the manual and ask good questions. We get good discussions. Even I just learned something!