A flip cap is the way to go especially for manually NUC'ing it. Butler Creek is good but IMHO the Vortex flip caps are a bit better. Unsure which one you need but easy to find out if you have digital calipers.

Regarding an external battery, we have a battery adapter from a discontinued product that will work for your purpose but you also need a USB battery pack. This adapter goes into the compartment and has a cable with USB-A connector.

The adapter isn't listed on our website so shoot me a PM or give me a ring. We do have a USB battery pack that mounts to picatinny but we are not allowed to post links so go to our site tnvc dot com and search "universal-rechargeable-battery-system". If you don't like that mounting method then are DIY options using other USB packs and other packs available elsewhere.