This mounting plate assembly originally came off a 180mm Maksutov made by Davro, consists of three 1/2" thick black-anodized aluminum plates. The two side plates 11.5x1.5" each have four counterbore holes where machine screws attached thru to the 8" OD tube body. The top surfaces of the plates are angled inward to match the curve of the tube. The base plate 10"x5" has an array of 12 thru holes for various mounting possibilites. There is plenty of room for drilling custom holes.
The white picture shows a similar 12" aperture version, the black photo is the actual part, looks nicer than the flash indicates.
$39 shipped in CONUS. Paypal encouraged.