Blocking the air intake should be a sure-fire way of killing the engine. I would try a phone book or similar - there is a good chance that the steel plate and the air intake are not parallel surfaces, which would leave a gap for air to pass through. I have heard of people using phone books for this purpose. Might be a good idea to duct tape 2 bands around it in both dimensions so the cover can't get sucked off and into the intake tract.

1988 Isuzu Trooper LS
2.8l Isuzu 4JB1-TC intercooled turbo diesel
www.dieseltrooper.blogspot.com