One idea 'roaming' in my head is that of contract propertariansim. It is an idea of what physical property really is (apart from a cloud of molecules) and how to manage it. There are a few points around which CP revolves:
1. All property is common property in reality:
Technically true, philosophically false. When you think about it, one individual and his family/supporters cannot fight off billions of people, if they were to try and take his/her property. This is not an argument for democracy, it is reality. The bigger group has a better chance of subduing the smaller one. Therefore, all property really belongs to all of humanity.
2. A person is entitled to own that which he and the people around him have agreed to:
From point #1, we can conclude that all 'private' property is really socialist property, where the property is 'created' by a mutual agreement between individuals. Each individual views the ownership of his property and the property claims of others as beneficial to him, hence why the property claims become legitimate. They would be illegitimate otherwise. Property is now mutually beneficial and agreed upon, meaning everyone benefits. The socialist part of this is that the property claim is collectively 'judged'.
3. All property agreements are contracts; mainly social ones:
If property is created by mutual consent between individuals, then this means it is nothing more and nothing less than a contract. Maybe not a physical one, but a contract nonetheless. In an anarchist society, a contract must be purely voluntary. That means no compulsion to enter, no force to maintain, you can always opt out and those who are not in the contract are not affected by it. As soon as a contract is involuntary, it is null and void. Same goes for property contracts.
In a property contract, all individuals who view the property claim as mutually beneficial and legitimate have 'signed' the contract and agreed to stay off said property. However, those who have not 'signed' the contract are not in any way obligated to follow it, or consider the claim legitimate. They have not agreed to the terms of the contract and the property is still available to them. This is not how typical propertarianism has been defined, where a property claim is universally legitimate if the proprietor has acquired the property in a 'legitimate' way. But legitimacy is in the eye of the judge, or in this case, the individuals involved in the claim. I believe this is better than the judge being some macho philosophers, who say this and that is legitimate because of some mystical link between an ego and a cloud of molecules a distance away.
4. Contract(ual) propertarianism fits liberty better than most other types of propertarianism:
First reason, unlike most other forms of propertarianism, CP is not based on arbitrary philosophical and/or rhetorical claims. CP is based on an agreement beween individuals at the time of or after the property claim; not on a prescriptive judgement of what is legitimate by a few philosophers before the claim.
Second reason, CP does not invovle the threat of coercion or violence. Since CP is based on a voluntary agreement, from which all parties see benefit, there is no (need for) any coercion or violence. There is not even need for violation consequences: why would anyone violate a contract, which they signed, seeing benefit from folowing the guidelines of that contract? Unless it some naive Stalinist who wants to toy with people, agree to stay off their pasture land, and then trample on it to make them angry for his amusement, I don't see a reason for one to violate a contract like this one.
The only other type of propertarianism I would say is compatible with liberty would be egoist propertarianism. Although rather vulgar and violent, it was actually the foundation for contractual propertarianism, which is merely EP without violence, secrecy and disorder.
