Actually, we already have a pretty good off-the-shelf material for sleeving a new Dorman upper. It is the thin-wall 321 stainless steel tubing from McMaster Carr (Part No. 6622K121) for about $25/ft delivered. Its outside diameter is .750" and the inside diameter is .710" So, with the reduced diameter pipe at .510 (?) an insulating gap of 0.100" would remain between the hot pipe and the heat shield. The outside diameter of the tubing is a little smaller than the EGR bore in the UIM, so it would need to be installed with high temperature copper RTV silicone to hold it in place. The silicone has a higher temperature rating than field tested JB weld, but would set up more quickly allowing for installation within say, an hour of installing the shield. If the shield were installed as the first step in changing out the UIM/LIM gaskets, it would be sufficiently set up by the time it was needed to be installed.
The best part of this solution is the price. A piece .820" in length can be provided for about $5. That is cheap enough for any L36 owner to buy a piece for the glovebox in anticipation of the day when the UIM will be changed out.
It would be better if we could find thin-wall tubing that would drive right into the bore, but the only products I have found thus far are incredibly expensive, like 10x the cost of the stock material from McMaster Carr.