Improvements

First of all, let me tell you how I prepare my salt fixer solution:
I use 300 g salt per liter of water. But, for one single bath, I don't need to prepare one liter, 500 ml is enough.

1. Weight 150 g of kitchen salt, yes, the grainy one. it's cheaper than table salt.
2. Put the salt in a heat resistent beaker and add hot water until you reach 550 ml.
3. Stir for several minutes, 10-20, until most of the salt is dissolved
4. Filter the solution with coffee filter and you will have about 500 ml of a concentrated salt solution. Filtering is important to avoid impurities but also to filter very small salt grains that will induce recrystalization of salt that may harm the film to be fixed.

This salt solution may be used as fixer but takes some 24 hours to fix at room temperature for common films and 48 hours or more for T grain films like TMax.

I challenged myself to find some additive that could make salt a better fixer, working faster. 

The first additive I discovered by trial and error was Potassium Bromide. Small amounts like 3 g/l will prevent recrystallization of salt and speed up the solution to some hours less.

Next stuff that went under my tests was Sodium Hypochlorite, common household bleach, less than a 5% solution of Sodium Hypochlorite. I knew, because already tried that a small amount of this bleach in water would eat the silver completely. But I thought that, maybe in a very small concentration, it could help salt to fix better.

Tests with stripes of film showed me that the idea was not silly, the film was cleared faster when I added some ml/l to the salt solution. At 5ml/l of bleach I could clear a stripe of TMax in 1 hour. So, maybe in 2 hours it fixs completely.

First result:

5ml/liter, fixed in 2 hours

I didn't count on this side effect, some white (in positive) spots that were produced by some deposits of some unknown material. On the other hand, the silver started to be dissolved too and the spots may be of silver 'condensate'. The film base was completely clear, with a yellow/brown stain that came out by washing for more than half an hour.

In my last experiment I reduced the amount of bleach to only 2 ml/liter and the fixing time to 4 hours. Waiting for the film to wash and dry. I will post in some hours... Be patient. I don't know how it will scan but looks better.

... waiting!

A little more time because refixing for more 2 hours was needed. Some typical vertical marks showed bad fixing in 4 hours. But for normal films could be more than sufficient, perhaps 2 or 3 hours would be enough.

... and waiting!

Here is the result, after 6 hours fixing of TMax400, expired 1991. The next I will experiment with Ilford FP4 or similar. I think that even after 6 hours TMax is not completely fixed. But I was affraid that the image could be eaten like the one before. This time no artifacts like stars in the sky and I will see how long I may fix to have a total fix. The film base was still slightly cloudy. The stripe will return to the fixer for some hours more. Maybe 10-12 hours in total which represents 4 times less than with salt alone. 
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Fixer used:

150 g salt
Hot Water to make 500 ml
2 g Potassium Bromide
1 ml of 5% solution of Sodium Hypochlorite