Been done many times by hiding the antenna inside the tank, & when done right the range is usually pretty acceptable for most people.
Here are links to just a couple example threads on it.

Antenna Mod repost

Internal Antenna

This is a picture of pzrwest coil internal antenna & a variation of it


The picture below shows a PZ III antenna located in the back with the antenna wire winding back & forth between the plastic braces.




Edited to add a few pictures.

~ Craig ~