By the Dark Gods! After making many many modifications, I finally generated a world that had the cubes. I finally found a gelationous cube in adventure mode when it was assigned to me as the creature to be killed. I was a Frost Giant axeman, and the thing just ate me, really, really quickly. It is no way a sliding target now.
The body:
[BODY:CUBE]
[BP:MB:body][STANCE][SIGHT][MOUTH]
[BP:FUL:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:FUM:body][STANCE][SIGHT][CON:MB]
[BP:FUR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:FML:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:FMM:body][STANCE][SIGHT][CON:MB]
[BP:FMR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:FLL:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:FLM:body][STANCE][SIGHT][CON:MB]
[BP:FLR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:MUL:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:MUM:body][STANCE][SIGHT][CON:MB]
[BP:MUR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:MML:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:MMR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:MBL:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:MBM:body][STANCE][SIGHT][CON:MB]
[BP:MBR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:RUL:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:RUM:body][STANCE][SIGHT][CON:MB]
[BP:RUR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:RML:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:RMM:body][STANCE][SIGHT][CON:MB]
[BP:RMR:body][STANCE][SIGHT][CON:MB][RIGHT]
[BP:RBL:body][STANCE][SIGHT][CON:MB][LEFT]
[BP:RBM:body][STANCE][SIGHT]
[BP:RBR:body][STANCE][SIGHT][CON:MB][RIGHT]
The actual creature:
[CREATURE:GELATINOUS_CUBE]
[NAME:gelatinous cube:gelatinous cubes:gelatinously cubic]
[TILE:'#'][COLOR:0:5:0]
[GENPOWER:3]
[CLUSTER_NUMBER:1:1][POPULATION_NUMBER:100:500]
[DAMBLOCK:100]
[BONECARN]
[NATURAL]
[BLOODTYPE:SLIME]
[LARGE_ROAMING][DIFFICULTY:3][FREQUENCY:1]
[LARGE_PREDATOR][MEANDERER]
[NOT_BUTCHERABLE]
[NOPAIN][EXTRAVISION][NOBREATHE][NOBLEED][NOSTUN][NONAUSEA][NOEMOTION]
[NOSTUCKINS][NOSKULL][NOSKIN][NOBONES][NOMEAT][NOTHOUGHT][NOEXERT]
[PREFSTRING:jelly-like consistency]
[PREFSTRING:burning acid]
[BODY:CUBE]
[SIZE:10]
[MODVALUE:30]
[PETVALUE:2000]
[EXTRACT:gelatinous cube poison:100:0:1]
[EXTRACT_VALUE:220]
[EXTRACT_PARALYZE]
[EXTRACT_ANTIDOTE:gelatinous cube antivenin:5:0:1:)]
[ATTACK:MAIN:BYTYPE:STANCE:slam:slams:1:2:GORE][ATTACKFLAG_CANLATCH][SPECIALATTACK_INJECT_EXTRACT:200:300]
[ATTACK:MAIN:BYTYPE:STANCE:engulf:engulfs:6:12:GORE][ATTACKFLAG_CANLATCH][SPECIALATTACK_SUCK_BLOOD:75:125][SPECIALATTACK_INJECT_EXTRACT:200:300]
[BIOME:SUBTERRANEAN_CHASM]
[BIOME:NOT_FREEZING]
[HOMEOTHERM:10067]
[LAYERING:10]
[SWIMS_LEARNED][SWIM_SPEED:2500]
[NO_SLEEP]
[CURIOUSBEAST_EATER]
[CURIOUSBEAST_GUZZLER]
[CURIOUSBEAST_ITEM]
[CANNOT_UNDEAD]
It rapes hard. I guess now the next step is to make a megabeast version :3