PC-50 Series Cylinder Port Single Pilot Operated Check Valve, 1/2 NPT Main Port, 1/4 NPT Pilot Port, Up To 16.00 GPM

It uses 1/2 NPT for the main port and 1/4 NPT for the pilot port.

Locking a cylinder against drift in neutral is exactly what this valve is built for. It blocks reverse flow out of the cylinder until pilot pressure is applied, so when your control valve is in neutral, nothing moves. One thing worth knowing: you'll need a 4-way control valve in the circuit for this to work correctly, and that goes for single-acting setups too, not just double-acting. The 4-way is what supplies the unlocking pressure to the pilot port when you want the cylinder to move. If your existing valve is already a 4-way, you're in good shape on that front.