So, I re-read your post and it sounds like you need a modern secure deadbolt of some sort. I'm guessing you want something more secure than a simple chain. If not then you can just install a swing bar lock like in hotel rooms. https://www.amazon.com/Prime-Line-Products-10308-Length-Diecast/dp/B003Z2NWNC

For something stronger, maybe a heftier swing lock. I installed two of the following on my front door. If you have the ability to drill into and secure to the door frame then maybe it will work for you. They open and close very easily with one hand and no stooping to the floor like some barricading products. I installed two just for extra security. https://www.amazon.com/Prime-Line-Products-10827-Reinforcement-Construction/dp/B00D2K367Y/

If the lock on the door is a deadbolt with knob-design on the interior side then consider something like this to prevent them from being able to turn the lock. https://www.amazon.com/Flip-Guard-FG-1000-SN/dp/B00M2KR1WG/

If you can drill into the floor and don't mind stooping then consider a floor-level barricade device. https://www.amazon.com/Nightlock-Security-Barricade-Bronze-Finish/dp/B008SX2FYA/