The crimson fist shoulder pad/upgrade kits would work, just paint the fist black instead of red.
Before I start, I recently discovered that fitting a sternhammer into a small list while retaining effectiveness can get a bit... tricky. Also, my group is at most semi-competitive, so take anything I say with a pinch of salt if looking for advice in competitive situations
Now that that's been said:
To take down buildings, imperial fist devastators get a tonne of bonuses in the sternhammer, so you cant go wrong there. In my experience a squad with basically any loadout will make buildings cry. With all of the bonuses they get I'm not convinced there is even an incorrect loadout to take! Tank hunter, siege masters and demolition expertise mean that almost any heavy weapon should wreck buildings or tanks in the hands of these guys - the only real issue is mobility - cataphractii captain maybe? Also they can get expensive for small games if you fill up on heavy weapons.
To take down tau: for a 750pt list there isn't much that can threaten a riptide (reliably), but for countering other threats
- thunderfire cannon or whirlwinds would let you shoot battlesuits even when they jump out of sight with their jump-shoot-jump antics, as well as destroying pathfinders in cover - also fluffy choice for fists. Damage output can range from amazing to a wet noodle depending on scatters, and not exactly durable, so try to hide them away from tau suits!
- vindicator is a cheap-ish way of fitting a big cannon into the skyhammer - can instagib anything the tau have, minus the big nasties
MCs and flyers. Main problem if you can't protect the side/rear armour it will die to even mild attention from tau shooting.
- For bigger games, grav centurions (or any relentless grav platform) is the only real counter marines have to riptides that I know of.
Never faced thousand sons, so can't help you there