heat

I use covers for each component when not in use.  Easily removed for playing time and easily put back into place after playing time (but only do so after the equipment has completely cooled down).  In order to have a nice look of fit & finish, I went to a local yardage shop and picked out a light weight and nice color of soft velvet.  I gave them the specifications for size that I wanted for each individual component (components vary in size).  They cut the pieces and sewed a box stitch around all edges.  These look as nice as the purpose and function they provide.  And low cost, too.  I hope this and other comments above help you in your quest.

Number one most important comment in the above posts, that you absolutely must pay attention to, is NEVER to play your equipment with a cover on it.  Ventilation during playing time is essential to product life, if not survival, including fire hazard, which could mean your own survival.

YES it is bad, ABSOLUTELY, NEVER DO IT!!!

Am I making myself clear?

Better yet, contact the manufacturer of your components and ask them.  There is a reason they built slots for ventilation in the component(s).  Covering them up, whether that be on the top, side, or bottom, is going against the grain of the engineering built into each of those units.  I promise you, that is what you will hear from them.