Although no longer in production, my 5904 (square trigger guard) rides in a Minute Man OWB that was designed for a Model 4006 and is so marked:
or in a Aker Flatsider Paddle:
I modified this holster by removing the paddle attachments (there are two pieces to the paddle attachment) and making a kydex belt loop, changing the angle so that it rides at a zero-degree cant. I prefer a vertical draw, 3:00 carry. The other modification that I did was to remove the thumb snap and reshape the top edges of the holster. This holster is designed for the Model 6906/Shorty Forty with a round trigger guard but, I found that it can be used (darned near perfectly!) for the 5904's square guard due to the location of the tension adjustment screw. Additionally, it keeps the muzzle end flush with the bottom opening of the holster without any protrusion.
I'm not certain of Aker still makes this model or not but their website shows a model D.A. 144 that they sell and lists that holster for both S&W models 4006 and a 4006 CHP 4006T.
I just happened to purchase mine new in the bag on eBay a few months back while shopping for a holster that would fit my Shorty Forty. I believe I paid $35.00 for it. Excellent holster with the mods I did on it!
Since you are looking for an OWB open top holster, might I suggest a Don Hume Model 721 OT? A seller has one listed on eBay currently (PM sent to you withe the link). I have one for my 1911 Compact and it is a very nicely constructed holster that conceals rather well. Not that expensive either:
Like many say, you'll probably wind up with a multitude of holsters for your pistol. Don't worry, it's normal and there is no cure. Ask me how I know this. *LOL*
Good luck in your holster search and congratulations on your 5904 purchase!
Welcome to the forum!
-Jim