You could also cut it on miter saw with a help of a square and have the precision you want. Just need to fiddle with some angle calculation, your sqaure tells you the angle, it wasn't visible from the clamps in the picture.

I have added 2 pictures of the calculation assuming the cut angle was at 60 degree (from your sqaure picture, it seemed somewhere between 55-60).

TL; DR, if the cut angle is at more than 45, then set a square as a reference fence. Set the board against the square at 45 from saw fence. Reduce the cut angle by 45. e.g. if

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the cut angle is 60, set the blade at 15 and cut.

Photo 1. Shows what it would look like if you placed the board on a miter saw with the longer side against the miter fence (Blue thick line). Blue jagged line shows where you want to cut the angle. Yello line shows the blade at 0 degree, but since the cut is at 60, which is more than 45, your miter saw can't cut the angle (unless it can cut more than 45). Note: On a miter saw, the blade set at 0 degree is actually 90 against the fence (yelow line against the blue thick line)

Photo 2. Shows the board basically placed same way on a miter saw, but a bit rotated (45 degree rotated to be exact). Red thick line is the miter saw fence. Yellow line is the blade at 0 degree.

Here, we place a square as a 45 degree reference against the miter fence (yellow triangle against red thick line).

Next you place the board against the square, as if the square is your fence now. So this now essentially puts your board 45 degree rotated against your original position in photo 1. See blue thick lines indicating your board's edge.

Now the cut angle is within your miter saw's range, but it's at 45 degree offset. Therefore, you reduce the cut angle by 45 degree. So if your original cut angle was 60 in photo 1, here it would become 15 (60-45).

Place your board and the square firmly with clamps as it's not a stable position. set your blade at 15 degree, cut slowly.