I, too, have been thinking about additional speakers, but I have been approaching it from a different angle. Since it has Bluetooth, I have been focusing on Bluetooth speakers. One of the inherent problems with many of them is the ability to mount them.
One of the first ones I tried was the Jabra Freeway. It is a great Bluetooth speaker and has very good reviews. It is not a good match for the Unication G5, though. It is much more suitable for continuous streaming of music or telephone calls than it is for two way radio communications monitoring.
I h ave also tried the Brookstone Big Blue Go Wireless Bluetooth Speaker. It comes with a suction cup mount and it about the size of many GPS devices. While it was somewhat more suitable than the JAbra Freeway, it still didn't quite do it for me for use with the Unication G5.
Currently I am using the EcoXGear EcoRox. One of the first things I should point out is it it is mountable. I am not a "camera guy", but I am learning that this speaker has a ¼ -20 camera mount screw post. According to Wikipedia, ".....most consumer cameras are fitted with 1/4-20 UNC threads." I should note that although not all Bluetooth portable speakers are mountable, The ones that I have seen that are seem to use the 1/4-20 camera mount screw post. Also, it should be noted that some companies that make equipment for mounting radios and other equipment, like Ram Mount and PanaVise, carry components that are compatible with the 1/4"-20 camera thread.
RAM 1" Ball with 1/4-20 Male Threaded Post for Cameras Unpackaged - RAM-B-237U | RAM Mounts PanaVise Products, Inc.
Anyway, back to the EcoXGear EcoRox. The MSRP is $129.99, which, in my not so humble opinion, is over the top expensive. However, Cabelas currently h as it on sale for $39.88. Also, it can be purchased through other retailers around the $30 to $40 range.
I am not necessarily totally set on the EcoXGear EcoRox. IT just happens to be the best one that I have found so far that works with the Unication G5. One of the other problems that I am having is finding detailed specifications for the other mountable Bluetooth speakers. Specifically, how many watts the speaker(s) is/are. In the specifications for the EcoXGear EcoRox is lists 8 watts (2 x 4 watt) speakers.
Top 10 Best Outdoor & Waterproof Bluetooth Speakers | Heavy.com