Clearance needed above hot water heater?

[3boxerss]

I have a 2021 Wildwood FSX 190RT
Im looking to put a small shelf in the empty space above the hot water heater.. Does anyone know what clearance is needed above and around the hot water heater? Thanks

[bubbles]

Typically the water heater is enclosed in foam and zero clearance from surrounding structures is acceptable. Rear access to the water heater needs to be maintained though.