The outfit that just composite-stocked my Springfield '03 did so in a way that prevents me from moving the magazine cutoff tab from the "On" position to the "Off" position.

The stock is much too petite and light to cut a slot in its side for the tab to move to the Off position.

So does anyone know of a handy-dandy way to modify the tab so the magazine cutoff does not work, but the bolt stop will still function?

Does anyone make a replacement tab I could install and use in this situation?

Yes. Have the idiots stockmaker that stocked your rifle fix it.

Can't you just leave the cut-off in the "on" position all the time? That's what I do on all of mine, have never wanted to cut the stock to allow for the "off" position.

Like these :

I can do that Craigster. Just like to have everything on a rifle capable of working. I don't want anything "half-working" if it can be totally disabled or fully enbled. Much less confusing in the field and, as I age, less confusing is a good thing.

Also, I think a smaller tab would look nicer.

AC, if the tab were smaller (depending on which way you mean smaller) there may not be enough meat left in it for the tapped hole for the detente, spring, and screw.

I'd wished that Springfield never put a magazine cutoff on it and went the Mauser route for the bolt release.