May be you can combine some models (or just use one model with the shape that you need) and then set POLYGON flag on it? You can customize BBOX by setting both FAT and NARROW flags on it, and then playing with it's MAX_X and MIN_X parameters:
Code:
my.eflags |= FAT | NARROW;
vec_set(my.min_x,vector(-50,-50,-50)); // play with -50 (X,Y,Z)
vec_set(my.max_x,vector(50,50,50)); // play with 50 (X,Y,Z)
