In my opinion, 2000mAh Lithium is pretty lame when compared to the radio's default capacity.
I decided to build one myself after realizing how much it was going to cost me to convert to Vertex's Li-ion battery plus charger. ?I used unprotected NCR18650b cells, which provide me with about 6 days of use on a single charge. The only thing I have to watch for is the discharge voltage. It is not good for the Li cells to run the pack down below 6V. With protected cells, the discharge voltage cut off is at about 3V per cell, so no worries there.
On my next build, I plan to use a protected 2S?Li-po with the proper dimensions to fix in the standard case to maintain water protection.
Here is a good how-to for those interested on building or converting old battery packs to Li-ion.
https://www.youtube.com/watch?v=AfAkIUUxPXk
I recommend these protected 3400mAh 18650 batteries(beware of cheap knock-offs)
https://www.amazon.com/Panasonic-NCR18650B-3400mAh-Protected-Batteries/dp/B00NUI47OY
Safety first...
http://batteryuniversity.com/learn/archive/lithium_ion_safety_concerns
