Here is a Knex target that I have made. As far as I am aware nobody else has used an elastic band in the exact way I have. (Yes people have used elastic bands for targets but not 'exactly' in this way. It is held up and can easily knocked down by a low power knex gun, as well as the high power ones. It is very important that it is on a flat surface otherwise it doesnt work. Also, the length of elastic band is important. Here it is! Feel free to rate and comment!

Step 1: Piece Count

5 yellow rods
5 blue rods
10 blue connectors
7 white rods
2 red triangular panels
4 grey/orange rods
1 green/red/biege rod
1 grey/purple half connectors
3 grey 2 socket, connectors
1 elastic band

Step 2: Bottom Half

1-2. Build this, closer View.
3-4.Build this, sorry for blurryness, different view.
5. Put yellow rod there.
6. Attach like this. Also put elastic band on where shown. You will probably need to experiment with the length of the elastic band, as I have.

Step 3: Top Half

1. Build these.
2. Attach as shown.
3-4. Now build these, attach as shown and put on the panels. (I had to use to triangular ones because I dont have one big panel.