You could have several exit doors that are locked and can only be opened from the inside... But anyone coming in, has to go through 1 guarded door. It's not rocket science... It can be done.
Banning guns would work better and would be cheaper, failure to register them for $100 a pop, would result in a $2000 fine, per gun and being caught with one in public would be a mandatory minimum. Registered and licensed owners would have no problem, for hunting, out in the sticks. Worked in all other liberal democracies that are just as free or more so than America, by any measure, regulating and restricting guns works amazingly well. 90% of folks will comply and get $100 or more for turning in their guns for destruction, if they don't want to bother with the expense of permits, licenses, trigger locks, safe storage and safety courses.
The solution has been found, it's all around you and it works to reduce gun deaths while increasing freedom by making democracy work better and society less fearful in general. You don't need a gun, you are not really under siege, they are not taking over (they are Americans too), though they might replace you, MAGA with better browner people.