Use what Pappenheimer suggested.
This saves the bitmaps as a cubemap instead of shot1, shot2 ect...

savedir="output";
var_nsave fh;
bmap* canvas;
bmap* b_render1;
bmap* b_render2;
bmap* b_render3;
bmap* b_render4;
bmap* b_render5;
bmap* b_render6;
var cubenumber = 0;
var directions[18] = 180, 0, 0, 90, 0, 0, 0, 0, 0, -90, 0, 0, 90, -90, 0, 90, 90, 0;
string tempstring1;
string tempstring2;
string _ts_;

//----------------------------------------------------------------------------- write_cubemap
function write8(byte) // write char
{
file_asc_write(fh, byte);
}

function write16(short) // write unsigned short
{
file_asc_write(fh, short&255);
file_asc_write(fh, (short>>8)&255);
}

function str_padding(str, number, padding)
{
str_for_num(_ts_, number);
var i = 0;
i = padding - str_len(_ts_);
while(i > 0)
{
str_cat(str, "0");
i-=1;
}
str_cat(str, _ts_);
}

function write_cubemap()
{
var i;
var xx;
var yy;
var format;
var pixel;
var pixelalpha;
var canvas_size[2];

canvas_size.x = bmap_width(b_render1);
canvas_size.y = bmap_height(b_render1);
format = bmap_lock(b_render1, 0);
bmap_lock(b_render2, 0);
bmap_lock(b_render3, 0);
bmap_lock(b_render4, 0);
bmap_lock(b_render5, 0);
bmap_lock(b_render6, 0);

str_cpy(tempstring1, "cubemap");
str_padding(tempstring1, cubenumber, 4);
str_cat(tempstring1, "+6.tga");
fh = file_open_write(tempstring1);
cubenumber+=1;
//--------------------------------------------------------write header
write8(0);
write8(0);
write8(2); // image type
write16(0);
write16(0);
write8(0);
write16(0);
write16(0);
write16(canvas_size.x * 6); // width
write16(canvas_size.y); // height
write8(24); // color depth
write8(0);
//--------------------------------------------------------write image data
yy = canvas_size.y - 1;
while(yy >= 0)
{
i = 0;
while(i < 6)
{
if(i==0){canvas=b_render1;}
if(i==1){canvas=b_render2;}
if(i==2){canvas=b_render3;}
if(i==3){canvas=b_render4;}
if(i==4){canvas=b_render5;}
if(i==5){canvas=b_render6;}
xx = 0;
while(xx < canvas_size.x)
{
pixel = pixel_for_bmap(canvas, xx, yy);
pixel_to_vec(temp, pixelalpha, format, pixel);
write8(temp.x); // b
write8(temp.y); // g
write8(temp.z); // r
xx+=1;
}
i+=1;
}
yy-=1;
}
file_close(fh);
bmap_unlock(b_render1);
bmap_unlock(b_render2);
bmap_unlock(b_render3);
bmap_unlock(b_render4);
bmap_unlock(b_render5);
bmap_unlock(b_render6);
}

//----------------------------------------------------------------------------- capture_cubemap
function capture_cubemap
{
var old_arc;
var old_x;
var old_y;
var old_screen;

b_render1 = bmap_create("render.tga"); // use a 256x256 tga for example -> determines cube map size
b_render2 = bmap_create("render.tga");
b_render3 = bmap_create("render.tga");
b_render4 = bmap_create("render.tga");
b_render5 = bmap_create("render.tga");
b_render6 = bmap_create("render.tga");

old_arc = camera.arc;
old_x = screen_size.x;
old_y = screen_size.y;
old_screen = video_screen;

camera.arc = 90;
video_set(256, 256, 32, 2); // should be same resolution as render.tga

freeze_mode = on;
vec_set(camera.pan, directions[0]); wait(1); bmap_for_screen(b_render1,1,0);
vec_set(camera.pan, directions[3]); wait(1); bmap_for_screen(b_render2,1,0);
vec_set(camera.pan, directions[6]); wait(1); bmap_for_screen(b_render3,1,0);
vec_set(camera.pan, directions[9]); wait(1); bmap_for_screen(b_render4,1,0);
vec_set(camera.pan, directions[12]); wait(1); bmap_for_screen(b_render5,1,0);
vec_set(camera.pan, directions[15]); wait(1); bmap_for_screen(b_render6,1,0);
freeze_mode = off;

wait(1);
write_cubemap();

wait(1);
camera.arc = old_arc;
video_set(old_x, old_y, 32, old_screen);
}

on_c=capture_cubemap;