I always soak the dowels and the holes they go into with thin CA, whether the fit is loose or not. Make sure your drip the CA onto the dowel endgrain, and watch the CA get sucked into it drop after drop until it will not accept any more. The CA does tend to swell the wood a bit and help tighten the fit. Also, the dowels filled with cured CA are also less likely to be compress by being repeatedly squeezed into and vibrated inside the holes. This technique does not, however, help with a dowel-hole fit that is very loose to begin with.