InvisiScript
Create an object, eg a simple cube.
Copy/paste the code below into a new script.
(Edit Object > Content > New script > Edit > Select All > Delete and then paste from clipboard).
Once saved the script will delete itself from the object's contents but the object will have invisiprim properties (and will be see through).
You can reshape the object and make copies.
string cloak = "38b86f85-2575-52a9-a531-23108d8da837";
default
{
state_entry()
{
llSetPrimitiveParams([PRIM_TEXTURE, ALL_SIDES, cloak, <0,0,0>, <0.475, 0.5, 0>, 0.0, PRIM_BUMP_SHINY, ALL_SIDES, PRIM_SHINY_NONE, PRIM_BUMP_BRIGHT]);
llRemoveInventory(llGetScriptName());
}
}