I just finished these, almost completely scratch built. One is 18mm for BT-55, the other is 24mm x 2.75" for BT-55. I used standard Estes engine hooks but I cut the little projecting loop and hook off of the bottom end so nothing hits the ground before the motor. I used 2 leftover solid centering rings for the 24mm mount. I cut rings from a cereal box for the 18mm mount. The 24mm engine block is a piece cut from the end of a used motor casing. The 18mm engine block is a strip of BT-60 glued and rolled into a smaller cylinder. Some Estes kits include a short piece of tube to slide over the engine hook to hold it in place. We've also had some discussion about engine hooks pulling out the back since the front of the hook is only bearing against one thickness of motor tube. So I cut some short pieces of scrap BT-20 and glue on on the motor tube before I put the engine hook on. Now the engine hook is pushing against 2 thickness of motor tube. Then I glue 2 of the rings of scrap over the outside of the motor hook to hold it in place.
