The classic "motte and bailey" fortification would still work in this situation. Basically a central building surrounded by a wall. Placement is key, it has to be up on a hill or other defensible earthwork. Since this can be made of wood, just a few people with chainsaws could construct it in a few weeks. If they have access to a small bulldozer or cat then they could heap up earth to form the motte very quickly. Of course using concrete or cinder-blocks would be better, but they would need access to those supplies. Since you want this to be a long term fortification, it needs to be placed where there is water, preferably a well INSIDE the walls so it will be secure in the event of a siege. Access needs to be controlled, either via an open and easily watched approach or by a double gate system (only one door is open at a time so visitors can be examined and searched prior to getting inside).
Another option, especially if the geography isn't favorable to the M&B, is something like Fort Caslop (build by the Lewis and Clarke expedition). Basically a C shaped building with a gate closing off the "C", making a protected courtyard. The exterior walls have narrow windows to allow for defensive fire. This was built by a handful of men with axes and saws, so well within the capability of your group.
Are guns/explosives still around? Explosives will make short work of anything with wooden, and even stone, walls. In that case you need sloped earthen walls that can absorb an explosive blast or at least deflect incoming solid projectiles. Guns mean snipers. So you will need to clear EVERYTHING from around your fortification. No trees, bushes, small buildings, NOTHING for at least 500 meters around. This way you can enter and leave with little risk of getting shot and no one can approach undetected.
Using an existing structure would save you a ton of time. Big buildings with few windows out in more remote areas are what you are looking for. Fortunately warehouses and distribution centers fit this criteria. Find where grocery stores and places like Sam's Club get their stuff. Usually some big building on the outskirts of a city, away from residential areas, with easy highway access. The buildings are large, have an open layout inside, and have few or no windows but have long loading bays with rolling doors. Then you can reinforce the entry points, guard the access road, and put observation windows wherever you want. Of course without electricity to run the A/C things will get hot in the summer but hopefully you have a little nuclear winter going on to help with that :) You can even add skylights to let in sunlight so you can grow plants indoors (assuming there is any sunlight). Collect the rain water runoff in cisterns (if it is potable and not filled with radioactive fallout).