Step 1

First draw a contour by placing the CPs near grid intersections.

Here the grid is 5cm spaced and the CPs are positioned such that the cube and the bevels have the dimensions of the primitive specifications. 

Note that at this point, it would be possible to tweek the gamma of each CP in order to obtain an almost flat splines where appropriate and rounded corners at the same time. However, it would mean that every time the object would need to be redimensioned, all those gammas would need to be re-tweeked to obtain the correct effect. To understand why, take a look at this explanation and also check this general discussion of Gammas in Animation Master. Using the method that follows means never have to tweek again.

Step 2

After selecting the whole contour, right-clicking "Snap to grid" and then clicking the "Peak" button, here is the figure we get.

At this point, we may decide to use flat bevels and continue assembling the model at step 4. But currently, we want to create rounded bevels.

Step 3

Adjust the gamma such that the bias handle is in line with the incoming spline, in this case, 45°.

The magnitude is also adjusted such that the curve of the bevel approximates a quarter of a circle. Through experimentation, I found that a value of 360 gives the correct result (note that in A:M v8.5, the required gamma value was 236).

Step 4

From a side view, move the outline at the z=20cm position. 

Copy-paste the contour and position it at z=-20cm.

This is what we get.

Step 5

Select all, copy-paste and rotate around the x-axis 90°.

Left-click-right-click each of the 8 intersecting CPs to glue them together.

This is what you get.

Step 6

Paste and rotate around the y-axis 90°.

Left-click-right-click each of the 16 remaining intersecting CPs to glue them together.

This is what you get. A beveled cube 50cm side with 5cm round bevels.