one trick I learned when dremeling out the rear turn signal holes was to take a shop vac (I have a small, 5 gallon one with a bunch of attachments), change it to blow mode and put a reducer on the hose that fit into one of the turn signal holes.
While dremeling one side you'll want to blow a lot of air into the other hole to force all the plastic debris back out the side your're working on. If you let all the plastic grindings get into the taillight unit you will have a b*tch of a time getting all the junk out. The taillight unit is sealed but the 2 turn signal parts are connected and any junk that gets in there can get trapped inside.
Here's a repost of a pic of my taillight assembly after enlarging the holes for the new bulbs and the bulbs, side by side. You can see the slight difference in width of the bulbs.