TO3 corrosion on power supply transistor

I just repaired a power supply for an infra red lamp used in a material guage, it had failed in a very unusual manner, the psu is placed in an under floor cable run, condensation is an issue.
The back of the TO3 transistor case has rusted right through and got to the die, the device is open so maybe its even rusted through the cat whisker to the pins.
Is there a stainless steel version of the 2n3055 available?
For now the replacement device is splodged in red oxide.

Could it be placed in a potting compound? Or pour a layer of silicone over it? Just a thought.

potting compound is a good idea, if I put the device through the bottom of a potting box then fill it up, sealed.
The tranny plugs in so if I drill holes right through after the compound has set it'd still be quickly replaceable.
Getting the holes in the right place is the only tricky bit.
I could make that on the bench ready then plug it in, looks like I'm working sunday on double bubble.

if there's enough room on the heatsink, you can see if a 2N3055 in a TO-3P case will fit and solder a wire from the C lead to a ring lug that would connect to the screw. the plastic case won't corrode, and the collector tab being tinned copper might be more resistant to corrosion. the TO-3P case was designed with the direct replacement of TO-3 metal cases in mind. the B and E leads are the same distance apart as on the metal case with the only difference being that they're not round. in the same manner TO-220 transistors are direct plug-in replacements for TO-66 metal transistors.

when clearance holes are needed in potting compound, the usual procedure is to use wood splints or small metal tubes into the places that need clearance. after the potting compound hardens, they are removed, and the clearance holes are there.

I could have done that, but its finished now.
I used 2n3055 h's, dunno what the h means, might be something to do with the way the substrate is fastened in.
I used short lengths of 4mm pneumatic tube for the screw holes, and left them in.