There are three parts to neutron shielding:
1) fast neutrons - hydrogen atoms in water or paraffin serve to slow down neutrons. The neutrons share their energy as they elastically scatter off the protons. The neutrons ending up equally sharing energy with the protons, “thermalized” to the room temperature energy. I haven’t seen an exact calculation for liquid hydrogen, but it has more H atoms per cubic meter than water, so the water shielding tables should be an approximation. The amount needed depends on the incident flux and acceptable fast-neutron leakage, but 3-8m should cover a lot of dynamic range.
2) Thermal neutrons are still dangerous, so you need to absorb them. Hydrogen has a small chance of absorbing them, but for water shields it’s best to mix in boron because of its large absorption probability. Boron also has interesting fusion aspects, so maybe it can plausibly mix with your fuel. But if not, put a few cm as a shield wall.
3) But you also need to stop the 2.2MeV gammas from neutron absorption on H. They’re unpleasant. If you have other need for gamma shielding, you can combine those. Otherwise, you’ll need either some high-Z material or a bunch of water. (It’s important to keep the fast neutrons out of that water, or you have to start all over with the thermal neutron and gamma shielding)
I can add numbers to this if you’ve got an estimate of the neutrons per second.