least known most fortified place for 300+ soldiers with supplies

Take a page from Blindspot and have them hole up in a disused bunker.

• in a forest/wooded area
• at least 1 mile away from any city/town and major roads

Many old bunkers and/or missile silos are in secluded areas.

• little known
• currently unoccupied

You could probably hand-wave this by saying it was built by someone that lost a war, or at least a government that no longer exists as such, and ended up forgotten. It would almost have to be unoccupied if it's little known, or else occupied by someone that you can take hostage (or worse, depending on how ruthless your rebels are) in order to take it over. OTOH, maybe one of your rebels is a crazy survivalist that is already occupying the bunker and offers it to his cohorts.

• fortified, or easily made to be
• has a few easily defensible entries/exits

Old, disused military installations are more or less by definition fortified with minimal, and defensible, accessways.

• hard to see on satellite. If it is seen, hard to see changes brought about by rebel occupation

Bunkers are typically designed to be hard to spot, not only from the air, but often from the ground also.

• accessible on foot, less so by vehicles, and preferably shielded from air attacks

Since they are mostly underground, bunkers offer protection from air attacks. If it's old, it may well not have any roads leading to it.

• able to contain supplies and living spaces for a few hundred rebels

If it's large enough, no problem, especially as bunkers are usually designed to be more-or-less self-sufficient for some period of time.

• near a natural water source(1 mile at most)

It's your world, you can easily hand-wave this.

• is not a cave. why? glad you asked.
  • confusing maze might get people lost in heat of combat
  • low ceilings aren't OSHA approved

A bunker can be as claustrophobic or as spacious as you like. Markings for finding your way around can likewise be as helpful or as confusing as you like, and some paint can change them.

• easy to cave in or block entries/exits
• rather dark, and lighting means electricity, which means generators, which mean fuel, which means going into town to keep the lights on
• way to easily besieged

Well, I can't really help with these, because they directly contradict your other requirements.

• preferably in an area that a local internet provider can hook up to internet

This is actually dumb. If it has internet hooked up by a local ISP, that's a paper trail that can be used to find you, and might even draw attention to the fact that you're making improvements, which contradicts your other requirements. Don't do that. Use a satellite hook-up, or long-range WiFi to another site in a more urban — and therefore less conspicuous — area.