JonTheMon and xnor's solutions assume we have superior equipment and skill, but the question states that we "have a hacksaw". Well with a hacksaw, we must start from an exposed side; we can't start a cut in the middle of a plank!

The most efficient "side" would be the current hole:

![practical solution][1] [1]: https://i.sstatic.net/hN7A3.png

If the hacksaw won't fit in the hole, we "cut an L shaped piece off the left and bottom", as stated first by JonTheMon.

JonTheMon and xnor's solutions assume we have superior equipment and skill, but the question states that we "have a hacksaw". Well with a hacksaw, we must start from an exposed side; we can't start a cut in the middle of a plank!

The most efficient "side" would be the current hole:

![practical solution][1] [1]: https://i.sstatic.net/hN7A3.png

If the hacksaw won't fit in the hole, we "cut an L shaped piece off the left and bottom", as stated first by JonTheMon.

Edit: I yield! GOTO0 found an even shorter practical solution.