If you're from europe, or just interested in alternatives to Testors Dullcoat, you could also use either Gunze's Mr Hobby Topcoat (which is acryl-based) or Tamiya's TS 80 flat coat spray (lacquer based - might change over time)...
I prefer the TS 80, it gives the mini a really flat finish, and if you apply more than one coat you won't even need a gloss coat underneath...
If you don't like spraycans you could also use a brush-on coat - from Vallejo (Matte Varnish, No. 192) for instance, just thin it down a bit, add a drop of dishwasher and a drop of isopropanolum and you won't see any brushstrokes...
P.S.: Don't spray when it's too hot or too humid - you will ruin your paintjob, and always shake your cans/paintbottles well before you use them - otherwise you'll get a shiny coat because the talcum powder (that's the flatbase) isn't well mixed with the varnish ...