I'm thinking of sending my injectors away for cleaning / servicing.

How should I seal up my engine from the elements while they are away? Won't I have open holes in my intake?

 

Stuff rags in the intake holes and cover the whole mess with a trash bag or other waterproof covering. Shut the hood.

While you wait, you might want to spend a little time with the plenum and runner flanges to get a better port match. It's probably the biggest single improvement you can make to the stock plenum and runners. Then you can open the ports behind the throttle body to take care of the #2 improvement.