I'm going to disagree with the other posters here and suggest you don't alter the grate at all. If the small coals can't fall through then when it starts to get full neither will the small bits of ash and you're going to run into a suffocation issue. You said the ash pan is about 1.5 inches below the grate and that kind of heat from coals will still help to keep the wood burning just fine.
You'd be better off looking at the type of wood you're using and how well it can keep the coal bed. In my region (eastern NC) the oak out here is terrible for a coal bed...rather than breaking down into smaller pieces it decays and gets soft and leaves almost no coal bed. So I know when I'm using oak I need to add in some pecan or hickory to keep that coal bed going. Pecan and hickory break down into many small pieces and keep that coal bed rocking along just fine.
As for temp management you've got a fairly small cook chamber with a large firebox and it's all well sealed. Once you get everything warmed up it's only going to take a small, energetic fire to keep the cook chamber up to temp. So you'll need to use smaller pieces of wood to keep temp where you want it, and at the same time allow for the replenishment of the coal bed as those small pieces burn down.
Just my suggestion but I certainly wouldn't jump into reducing the airflow around your burning wood. The design of this cooker only allows air into the firebox from one angle, ie the door damper, so if you have wind going any other way it's going to force the drafting system to work a bit harder to get air into the firebox. If you start reducing the airflow by keeping too much ash in the coal bed you're running a risk of suffocating the fire, and that's certainly not what you want to do.
Good luck and beautiful cooker...LSG did a good job with the re-design and the price point as well.
