A guy in my club just goes to the store, buys an oil funnel, cuts the point off, and hangs it onto the bottom of a BT!
You could also use the template widget and make a transition for whatever your BT's OD is to however large you want your shroud to be. Print it on card stock, add a joint strip for overlapping, and then coat the inside with some thin epoxy to harden it, slide it into place and let it dry. (Hanging it upside down will let the epoxy run up into the shroud/bt joint and make it solid.)
WW