How do you design downfiring boxes?

I’m trying to design a one sided trapezoid box that is downfiring to look more stealth. But I can’t seem to figure out how to get the sides to come down and create feet. This way the cuts would also include this element. 

I don't quite understand, what did you mean? Can you explain please? Below you can see the screenshot with one side trapezoid

With a downfiring box, there needs to be space underneath the woofer and the floor to allow the sound to escape. Sort of like this: https://www.amazon.com/Speaker-Box-Enclosure-Downfire-Subwoofer/dp/B076V5ZTTW 

This would impact the length of the side panels to create a lift off the floor. 

Now I understand what you meant. There is no such option but it may be a great feature. Is it used frequently or rare?

I can only suggest cutting these platforms separately and then glue them to the box

It is a much desired feature. I purchased the software to build this exact downfire box. But now I have my answer. I have been trying to build it for a few days in the software. I believe Term Pro allows it in their software. I will try it and let you know.