Friend of ours, now out of boating, had a rack built for his two 9' kayaks.
These are the best shot I have of the setup as I was not taking the shots for this purpose.
It was built of aluminum, square 3" tube , two arms carpet covered for each kayak with tie down provision..
The kayaks overhung the walkaround at the back so interfered somewhat but they were still able to get around.
His boat was a 40' tricabin so your boat should be able to handle somethign similar depending of course on kayak size.
I have no idea what the cost was so can't pffer any guidance there.