The firewall hole is there for a reason. If you block off that hole all the air entering the engine compartment to feed the fan and carburetor(s) will have to come through the grille on the engine lid. If you look at where the air actually comes through the engine lid you will see that there's not very much square footage for airflow- less than half (12-15 sq. in. in some Speedster engine lids compared to 30 sq. in. below the rear window on a Beetle) of a stock Beetle. When VW went to the wider 'doghouse' fan they put louvers in the engine lid itself, increasing air intake again. The Beetle engine lids with 4 sets of louvers effectively double the air intake area, so if the only air entering the engine compartment is coming through the middle of the engine lid on your Speedster you can see why people are not recommending you cover the firewall hole. Oh- and dual carbs draw even more air.