After all, it is a simple box construction and it should be fairly easy to draw up.
Yes, it is a simple box. Just make the darned thing. Faster than messing about with software.
What height do you want?
What length do you want?
What width do you want?
Add 2 * length + 2 * width, and a few inches for oop's to determine the length of the board.
Rip the board to the height you want on the table saw.
Using the mitre gauge, set the blade to 445 deg.
Make the first 45 deg cut. Flip the board over and measure for e.g., a width, mark the cut and the blade width. Make the next cut. Now you have the first end. Mark this piece End "A" for example.
Flip the wood again, now measure for a length, mark the cut and the blade width. Make the next cut. Mark this piece "Side" B and the edge from the first cut "A". Now those two edges should be a perfect 90 deg.
Flip the wood again, now measure for a width. Mark the cut and the blade width. Make the next cut. Mark this piece End "C".
Flip the wood again, now measure for the last length. Mark the cut and the blade width. Make the next cut. Mark this piece side "D".
Flip the wood again for the last cut. You will likely have to also flip end for end unless the board is really long.
Now the only two edges which may need tweaking to be perfect fit should be the first cut on "A" and the last cut on "D"
Test the fit of the mitres. Tweak as needed.
Cut a groove close to one set of edges for the bottom. Width of groove depends on the material you are using.
Do you get the idea? Basic dimensions, then just begin to make the pieces.