This is based on my previous tv cabinet design. but shortened the centre supports, its now only at the back and supporting the weight of the TV, the front of the supports has been replaced with suspended bolts from top panel.
Checkout the pic below and its pretty much self explanatory.
The bolt I have used is M12 x 240mm stainless steel bolt form Bunnings, they are available in all sizes, maximum is 300mm, whch translates to 200mm - 240mm adjustable range.
The adjustable height of the platform at the moment with 240mm bolt is between 140mm to 185mm, if you bought another centre speaker taller than 185mm, you can always go down to bunnings to get these bolts, they are like $8 or $12 each.
Probably one of the first cabinet on the market with adjustable platform for the centre speaker?



