I usually always recommend against building huge passive reactors(without turbines) as it is so inefficient. Turbines are approx 10x as efficient(as long as you have a constant power need).
Without Cryotheum your best and only option is Ender. You usually want to use those two in combination as they are mutually opposites. But I tried putting together a few example reactors using only Ender:
http://br.sidoh.org/#reactor-design...ertion=87&layout=O5EOE5X2E5X2E5X2E5X2E5XEO5EO
http://br.sidoh.org/#reactor-design...X2EXEXEXEX2E7X2EXEXEXEX2E7X2EXEXEXEX2E7XEO7EO
Now, I design reactors with the mindset of: Give me a volume of space and I will strive to design the best possible reactor in that space. This is because I always incorporate my reactors into builds.
It is also very important to know the size of reactor, since the placements of conductive materials and fuel rods needs to be optimized against that size. If you change the size of a reactor then you cannot simply use the same design as a different sized reactor and expect it to hit the "sweet spot" as well.
Note: Some people will argue that you need multiple layers of Ender/whatever on the outside of the fuel rods to catch radiation. This is a sound idea as this is how the mechanic work. But in reality in every single case I have seen, it have always been more efficient to use that space for more fuel rods(irradiate each other for increased fuel efficiency) and cooling there of.
If you end up finding a specific reactor size you would prefer, just post it and I can give it another go.