How to indicate that "I need more time to consider your draw offer" in a formal game? When my opponent makes a draw offer,  I will consider whether to accept this draw offer or not and this consideration could take very long, say 20 minutes or more. 

What is a good etiquette now to tell my opponent that I am neither declining nor accepting this draw offer; instead, I simply need more time to consider it?

To tell my opponent to "wait"? Or simply remain silent while considering? Or something else?
The polite way to react to a legally made (opponent moves, offers draw and then presses the clock) draw offer which you are not going to immediately accept is to say something like "I'll think about it".

In a team competitions it is quite common for the player offered the draw to stand up and go and look at his teammates' games to see the standing in the match before making a decision which affects team success. This is allowed even though it is the player's turn to move. You are not glued to your seat when it is your turn but you must not leave the "playing area" - generally the room where the event is taking place. You may even ask your team captain if you may accept the draw (unless the competition rules specifically forbid this). You may follow or reject your captain's advice.

It is also acceptable, though less polite, to completely ignore your opponent as you may be tempted to do if they are offering a draw in a lost position.