dowel pins & corresponding drill sizes to get a tight fit....

[birchy]

Some drills will cut under size, it usually depends on how they are ground. Cheap drill bits are often slightly under. With plastic, if you drill it at a slowish speed and high feed (thus avoiding the drill from spinning in the hole for too long) the plastic will usually "stretch" slightly and produce a hole slightly smaller than the drill bit dia. The other option is to drill something like a 1.5mm hole, heat the pin to around 150C with a paint stripper and push it into the hole. Experiment with offcuts first to get a feel for the best technique!

EDIT:
Seeing as you already have "snug" fitting holes, you could also give the end of the pin a light tap with a small ball pein hammer to make them slightly oval so they fit tighter.