Following is what I found about this design.

There are three different horn designs that can be implemented:

1- Conventional design

2- Folded horn

3- Dog-bone horn

Folded and Dog-bone designs seemed too complicated and are meant for more precise ultrasonic horns (used mostly for welding).

Here are two designs for the conventional horns. Typically the length of the horn should be half of the wavelength of used frequency.

(This is from a catalog of horns used mostly in welding)

(This is a more simple design that works better for my purposes)

Now, I just have to have this made at the mechanical shop and see how effective it will be.

Here are some other useful webpages:

Plastic Welding Horns

Novel Horn Designs (Very good paper)