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I found that in order to have a Shimano "ultra-width" 6s freewheel that I had to start with a 7s Dura-Ace freewheel, remove the small cog and then force a modified plastic shield in from a 600 6s freewheel in order to eliminate the huge gap that left the bearings exposed.
I've also used an O-ring to close off the opening left by removing the smallest externally-threaded cog from a Shimano Dura-Ace or Sante 7s freewheel.

Was yours also a modified 7s unit, or did you perhaps mean ...Suntour?

For hubs like certain vintages of Phil Wood where the (fixed) 5s axle spacing may not allow even an Ultra-6 freewheel to quite fit in the space, I built a 13-24t five-speed Uniglide freewheel to allow tight-enough ratios and sufficient range for spirited riding here in the foothills.
Combined with a 52-36t chainset, it achieves the same low gear ratio as 42-28t and has a decent 52-13t top gear (shown below).

On most 5s-era bikes, I have no trouble re-spacing the axle to 122m to accomodate a standard-spaced 6s freewheel. Atom/Normandy hubs in particular need only the slightest of spacer width changes to accommodate standard 6s Shimano freewheels, as long as the frame isn't one with inwardly-protruding seatstay ends (though going from a 14t to 13t top cog really improves the clearance there).

Clearance for inwardly-protruding axle stop or claw hanger hardware can usually be achieved using a Dremel where needed (on bikes without screw adjusters and/or without integral derailer hangers). Single axle stops are best moved to the left dropout (only one is really needed) and claw mounts serve as an axle stop (so no discreet stop is needed).