you could build a mdf or wood box that covers the transformer, then line the inside of the box with copper foil or copper sheets. if you haven't noticed any ill effects due to the transformer being in your room, i'd say just leave well enough alone.
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I own 6 of these iso-trannis and never noticed any negative effect in a low power SET/high(ish) efficiency system.
But if there turns out to be a problem, simple to a search for "mu-metal" and either buy a mu bag or ox to place over the Topaz. Mu-Metal was created specifically to block stray magnetic fields from motors, magnets, transformers, etc.
Check with Mapman here on Audiogon, I recommended he use some for hum he was getting whenever his phono stepup amp was near other components. It solved the problem for a just a few bucks
The EMF Safety Superstore sells several different types of Mu-Metal or equivalent shielding products from $2.50 per linear foot to super expensive. I'm sure they could recommend just thing you need.
Gauss Meter sells minimum orders of 3 ft @ $24 per. (half-way down page)
I have a 4kva Topaz iso and if my memory serves me correctly, there is a separately grounded shield on both the primary and secondary windings. This serves to reduce coupling between them and would therefore reduce any magnetic fields as well. While transformers use coils which generate magnetic fields, the Topaz is not an ordinary transformer in this regard.