Having taken apart a few power blocks, they all use a large aluminia insulator, with the power devices soldered to the ceramic on silver plated pads, and the bottom side soldered to a large copper heat spreader. Good for 2kV of isolation, and very reliable.
You probably will want to get large alumina plates ( at least a lot larger than the devices for creepage clearance), and use a spring clip or a top clamp with Belleville washers to provide the correct spring clamping to the devices. Then appropriate thermal compound and the manufacturer of the devices recommended clamping force per device. Note with the aluminia isolator you do need to have a machined burr free heatsink mounting surface, as extruded is not going to work, it has to be machined to be as close tolerance as the ground aluminia surface to prevent cracking. Remember that the clamp as well needs to be as rigid as the heatsink, so as to apply even pressure, so you will likely either have a bar across the top ( probably with a kapton tape to provide higher isolation) and a clamp assembly both ends, or a longer bar with multiple devices being clamped by it. When doing multiple devices with a single clamp make sure they are all from the same lot number, as they might have slight differences in the thickness of the top epoxy, and tolerances add up.
https://en.wikipedia.org/wiki/Belleville_washerBasic info, use with a nylock nut to provide the anti loosen requirement, the washer stack provides the even clamping force with changes in operating temperature.
http://www.bellevillesprings.com/belleville-washers.htmlUK company, I have bought from them before, good service even international, and very friendly as well. Good quality items.
Worst thing are power SCR pucks, they have a special heatsink assembly that you must use to get the lifetime, and generally no warranty if you do not use it either or use incorrectly. Those things do not let magic smoke out, they instead turn into shrapnel powder with a loud bang, then blow the expensive FF semiconductor fuse.