I used to have a lot of trouble ironing Monokote into cockpits and other small places where a standard iron will not fit.
You can buy a tiny iron called a “trim sealing tool”. I had one of these for a short time, but the temperature control was inaccurate, and the shape was not very helpful. So I got rid of it.
A better solution to this problem is a butter knife. Heat it on your iron for about 30 seconds and use it like an iron. It could also be heated with a heat gun, but it takes a bit longer. Heat is transferred a lot better by contact with the metal iron.
The knife will fit into tiny spaces where an iron won’t, and it will reach a lot farther than the trim irons that you find in the hobby store. I used a butter knife to iron the entire cockpit of this Small Wonder built from RCM plans.
You can try the same trick with a spoon for ironing fillets and other concave surfaces. Spoons have to be heated with a heat gun rather than an iron, usually.