Jim;
Here's a link to give you some ideas on how to make one for yourself. You can use either 1-1/4" or 1-1/2" emt conduit coupler and a 90 degree elbow (depending on the size of the exhaust pipe from your generator). Some 1/2" wide flat bar stock, 3" PVC or ABS plastic pipe, a coupler or 2 for the 3" pipe and a couple of pop rivets or self-tapping screws.
I made mine from ABS (black) pipe because it is lighter than white PVC. After a couple of years I drilled out the rivets I used and replaced them with self tapping screws because the rivets loosened up from the heating and cooling down.
You will see in the pictures that the PVC pipe is positioned about 1" above the end of the emt elbow. This is necessary to draw cool air up into the plastic pipe behind the warm air of the exhaust and keep the plastic pipe from becoming too warm.
http://showland.org/rv_mods/genturi/genturi.html
Hope this helps you.
Hey Dan Moore, you're #1.
RIP Mark Graham