try 2 bits of tube whatever diameter you want, cut them to length and then fix them to whatever centres you require.
then span a piece of 18mm MDF board, fix using self tappers. then mark out the vesa centres thats required for the TV and drill (normally M8's) and then bolt the TV to your new bracket. The Trick is everything needs to be spot on.
I am in the throes of designing a tv stand where the TV pops up from out of the cabinet via remote control. won't be building it for a while though as its for the mother in law and she isn't a paying customer.