I usually have this problem with the knurled pots from Tayda. You can either put an additional nut on the inside of the enclosure or just do what I do. File down the shafts of the pots a few mm. BTW, you can get nuts for pots from Mammoth Electronics. One time I bought a bunch of wrong pots which I'll never use so I have plenty spare nuts and washers. Oh well!
aka Dead Eye