The Bisson Pitts is a smaller volume muffler than the Slimline, hence I have one fitted inside the cowl of my BH EXtra 300s.
For the Venus you will not get a Pitts inside it, the cowl is too narrow. Mounting the stock muffler is probably best.
Also look at
www.justengines.unseen.org and have a look at their range. You may be able to get away with an Inverted Upright Muffler and they do custom mufflers as well.