For stability, which is important, I drilled a couple of holes at the near bottom and fastened the bottom shelf with screws.

I also put in some of the other shelfs. Depending on how stable your contruction is you may have to screw them tightly to the sides as well.

Finishing

Some of the remaning back pieces I slightly bended and put at the front. Not that it's needed but you're allowed to get creative no?

Stability and vibrations

What you could do though is put some weight into those spaces to make it even more stable.

And you will need something to dampen the vibrations so they don't transmit into the table. Either between the speaker and the stand or the stand and the table.
My speakers however already came with dampeners.

Borgstation
Just decorating but could be used for covering extra weight if needed.

That's that.

May it inspire you to find your own solution