I'm thinking about a Commando style carbine with an Osprey/Bushmaster/Ares style retrofit.

Anybody else tried modding stock handguards to fit?

This is neat:607 modern

Osprey will be the least compatible of those. Ares and bushmaster would be the best, however I don't think you'll be able to find either of those anywhere

How about a mock dissipator with a superlative arms?

Stock handguards (round or triangular) will not fit over an Ares Defense / Bushmaster piston kit without severe modification to provide clearance for the piston housing.  Additionally the stock heatshields will interfere.  The Ares kits are still available from Ares Defense.  The Ares kit is delivered with a set of M4 Carbine style handguards that have been designed and manufactured to provide the necessary clearance for the piston housing.

Superlative Arms would be the most compAtible retrofit. I have a mock Sissy under a BCM KMR. While it's a tight fit it still works.

I'm willing to cut up some heat shields and then some.

for the osprey 416 the handguards come with the kit and all that has to be done on regular M4 handguards is where the gas tube tract is you widen that to fit over the gas cyclinder,,,,magpul makes carbine,midlegnth and full size that will fit over a piston system

It also has surprisingly better recoil management than my other DI rifles, which is interesting.

Magpul handguard is compatible with Ares, Bushmaster and Adam Arms piston.

It can be done for the Osprey piston, just need to machine the heatshield each side by .250".
This way the snout piston part sliding/moving freely.

Osprey fits under skinny round guards if you remove the upper heat shield and dremel out a small amount of plastic.  I've had two 5.45's with skinnys since the early days of Osprey when they used a bolt on gas key, since then they've revised to a 1 piece machined carrier with integrated key.  Rumor was around 6000-10,000 rounds some gas key Allen heads were breaking, so they eliminated that flaw.  I love the setup! I did not use the fat M4 diameter guards the kits came with. Sorry no pics, my stuff is still in hibernation until spring ;<)
Keep in mind the longer your barrel the smoother your action will be since the Gas port is further forward, adding a slight pressure reduction and time delay. My 16 inch carbine actions "feel" much more mechanical and clunky compared to DI before the conversion.  I think this is true with any of these piston systems. Totally worth it on 5.45 with corrosive ammo since the BCG stays so clean!  Oil and grease stay put too.