Why does it say "va'Ya'asu"? In Melachim II, 12:10 it says "va'Yikach"!
Radak: This teaches that they made a hole in the door 1 , like it says in Melachim II, 12:10.
There it discusses the box for new [half]-Shekalim. There already was a Shofar in the Mikdash for this (they did not need make a new box). Here it discusses a box for old [half]-Shekalim. Even though there was a Shofar for this, they made a new box. (Malbim here and in Melachim implies like this. Perhaps the Shofar could not hold all the money they would collect now. - PF)
Why does it say "Aron Echad"?
Radak: It means Aron Ish Echad. 1
Refer to Melachim II, 12:10:1:2 and the note there.
Refer to Melachim II, 12:10:1:1*.
Why did they put it outside Sha'ar Beis Hashem?
'Rashi': This is so any generous person could come 1 and give.
I.e. even if he was Tamei, and could not enter. (PF)
Were they not concerned lest a swindler steal coins from the Aron?
'Rashi': Melachim II, 12:10 says that they made a hole in the door of the Aron so people could cast coins inside, but no one could take from it.
Malbim implies that there was no hole in this Aron. It seems that they were not concerned for thieves. (PF)
Here it says that the box was put outside Sha'ar Beis Hashem. Melachim II, 12:10 says that they put it by the Mizbe'ach!
Malbim: They put a box by the Mizbe'ach to collect Shekalim for this year. They put another box in the gate, outside, for extra Shekalim to compensate for previous years [refer to 24:7:2:2).
