I am using here the same Octagon Twin OTSLO with 2nd output modified for GPSDO input 27,841846MHz just using a second coax and producing an IF of 435MHz with kHz the same as on the SAT downlink. All experimental setup with GPSDO outside next to the RX-dish (weatherproofed).
Modification after all was very easy. No problem when you even remove the x-tal without de-soldering. In my case it ripped-off when trying to use a Dremel tool. The large solderpads are not used and very simple adding a 1nF chip-C for DC blocking, a 51 ohm load* and a short enamelled (isolated) copper CUL wire as shown.
A small notch needs to be made in the cover part, preventing the wire getting pinched-off....
*IMPORTANT UPDATE: My modified LNB lately suffered from sensitivity issue at startup. I finally did find out that the 50 ohms load was placed wrongly (as seen on last picture). The RDA3565 PLL circuit was possibly loaded too much with the 50 ohm (DC-)connected directly to the PLL-pin. Solution was to remove the 50 ohm, because the feeding coax is very short in my case. See the second picture with only extra C = 10nF that provides correct operation!.
When a 50 ohm load is needed, please change the place of the DC-decoupling C AFTER the load, seen from the input. This is visible in several other modifications.
Apologies, to whom following my initial pictures for modification...
73 Ed PA1EW