Here's my explanation for what is needed to make one of these with corrected dwg. below.
1.) On your lathe, measure the inside distance between ways (the top surface of the bed).
2.) Measure the distance between the shears (the vertical supporting surfaces of the bed)
3.) If a bandsaw is available, set the miter gauge over to 75 deg.
4.) Cut one end of a piece of 2" X 1/2" (50 x 12mm) bar stock to that angle.
5.) Mark on the stock, square to the cut edge, the distance between the shears you got earlier, minus a small amount for clearance.
6.) Make your second angled cut. This yields the rough piece. A parallelogram faced prism.
7.) Measure across the closer corners on your piece.
8.) Subtract from that the distance between the ways you got earlier, plus a little more for clearance..
9.) That gives you the amount you need to mill off of the two corners. Divide that by 2 to get the amount you need to mill off each corner.
10.) Set the part up in your mill vise, adjusting the long edge of your part against a combination square's 45 degree edge to clamp it at that angle.
11.) Mill down to your depth figure (in step 9).
12.) Flip the part and mill down the other corner to your figure. Done.