Here's a couple pics of mine. Just 3/16" plate cut to the right shape and tack-hammer, tack-hammer....... till its flush, then welded in solid. Also cut out drain and plated the bottom.
Cost: 2 dollars + 2 hours
I am another of pygpen's satisfied customers. best bang for the buck if you are looking for something to just weld on. If you want to fab something up there are a few good ideas on here. If you can find the 10" pipe cap and want to do the cutting yourself, don't forget to notch for the drain on the bottom.
I've got a plasma and found a couple local vendors for the 10"/8" pipe caps, anyone interested in one? I checked with pygpen and he is apparently no longer producing the diff protection.
I welded a piece of 1.5" x 1/4" thick bent to the radius of the ring gear . pretty simple but it works well . the ring gear is the only thing that is close to either cover front or rear .
If you were to drain the fluid, I would expect that to "burn" more fluid since there would only be a thin residue vs a gallon of fluid to dissipate the heat throughout. Don't see how it could affect gears etc.