Very simply, I built a plywood box that had the same dimensions of the old TV.
Using spray-on adhesive, I covered the box using a cheap $5 black blanket from Walmart. The blanket was cheaper than buying black felt.
I attached the new TV to box using four mounting bolts. Make sure that the bolts match the thread pattern of back of the TV and make sure that they aren't too long or too short.
I slid the black box (with the attached TV) into the old TV cavity and secured the box using same method that the old TV was secured.
The back TV was even easier, It was strapped down to the top of the dresser. I removed the old TV and mounted a new one to the corner of the room using nothing more than single black piece of plywood. The TV just seems to float there in the corner and (other than four mounting bolts) no expensive mounting hardware was purchased.
'93 Itasca Suncrusier diesel towing a '05 Honda CR-V.
Bob, Pam(DW), Bridget(DD) and Christine(DD)
See you at most of the Penn State tailgates:)