Some concerns, you have to use some compartment ventilation no matter what, especially if the tank is portable and vents inside but if the tank fills and vents outside the compartment then I don't see a problem having only some vents rather than the unwanted mesh type back doors. Other than fuel line permeation for vapors the only other cause would be a leak however small. So you need to come up with some sort of compartment ventilation and not have it sealed up..
IO's have ALL the fuel connections enclosed and that's why they are required to have a blower. It's all about fuel line connections and ventilation. Having an ignition source is something to worry about with fuel vapors as with an IO backfiring, starter, alternator and such. Nothing other than a battery with secure connections being in the compartment won't make your boat go boom without a blower if it's an OB.
Personally I would find some sort of SS vent grate to add in to allow some fresh air to enter and push out any vapors that may collect.
