Since drag cubes are a cursed insanity rune and an alternate solution to this isn't likely to be forthcoming soon, just add a small amount of Ablator to the Mercury and Gemini command modules.

Tested, it seems to keep them just under their skin temperature limits even in the event of a worse-case scenario (steep reentry and intentionally bad pointing), without allowing them to re-enter without a heat shield.

Also slightly increase the ablator load in the Mercury Heat Shield because it tends to run out even on a normal re-entry.