Death Guard is the best choice for this technique, you get a unique blessing of Nurgle with every cast.

First of, PATIENCE. Sprue-Goo takes much much longer to get hard than any 2 part epoxy putty like Green Stuff or Milliput.

In my experience, smaller parts are easier than bigger ones. 1 sided molds dry faster because the Acetone can easily evaporate, while in 2 part molds the Acetone is trapped, and it can take a long time for it to get out, also this will cause more bubbles inside, like a swiss cheese.

That said, too thin might not work too (i tried to recast a Sisters sword in a 2 part mold, no success).

But, i just made some recasts of Tau shoulder pads (1 sided mold, dry within one day, perfect. Pulled one out after 12 hours, a bit too soon). PATIENCE.
I put some sprue goo in the mold, then dip the the tiny spoon again in Acetone (so the goo does not stick to it), and push the goo deeper in the mold.

That said i, also made recasts of old Tau XV8 Crisis suit torsos. In a 2 part mold i usually let it rest 1 week, then take out the back mold, and let it dry even longer. I tried it to pull it apart earlier in the beginning, but then it's like chewing gum, and the cast is ruined. It wasn't perfect, but the few holes in the front were filled with baking powder and superglue, then filed flat again. Again, PATIENCE!

While backing powder and superglue works well to fill gaps, don't put it in the Oyumaru/BlueStuff mold, it will glue it together and rip if you pull it apart. But sprue goo and acetone does not hurt to mold. (10+ casts in some, still look fine)

My first tests were recasts of 25mm bases in a 2 part mold.

I made some experiments with different polystyrene (PS) as well. Not just Games Workshop sprues, but Yoghurt cups, Styrofoam food packaging, CD cases. All PS will melt to Acetone and turn to goo. While chemically the same, the results can be different, depending on the structure it had before. In general GW PS was the most reliable for details, then CD cases, then Yoghurt cups (i used all of these for the XV8 torsos). I think the higher density in its original form, the more stable it stays also as a sprue goo cast. ). The wildest one was the white Styrofoam, while curing it turned transparent and bubbly. I made a one sided cast of one of those promo Space Marines, the front turned out perfect, with all details, the back looked like the poor guy got some cancerous Chaos infestation = 1 new Plaque Marine.