Measure the size of your television from front to back.
Add a few inches to your measurement for safety, then multiply it by 2.
Cut a square out of MDF, with a diagonal measurement (corner to corner) matching the number you calculated in step 2.
Cut the square in half along the diagonal, creating two equal triangles. They will both be large enough to hold your television, with the extra inches you added for safety.
Cut four pieces of 2-by-4 to the desired height of your TV unit.
Assemble your TV unit by placing one triangle on the floor. Stand the 2-by-4 pieces onto it, two in the back corner and one on either front corner. Put the second triangle on top to complete the unit.
Drill pilot holes and screw all of the pieces together.
Paint or stain the unit to match the other furniture in the room. Set the TV on top and store other media devices inside the unit.