Here is the size you need but this is only for a basic spacer that is not made to also support the dial.
28.5mm outside diameter (fits rolex cases made for 1520/1570 etc)
26.0mm inside diameter (fits eta 28xx movements)
Approximately 2.25mm thick depending on if case clamps with screws are used or with screws fitting into a groove cut in the case. A dedicated 2824 spacer may be thinner than 2.25mm, the spacer I measured was made for an eta 2836.
Most standard brass eta spacers are 29.0mm od x 26.0mm id x 2.3mm thick and can easily be modified to work in your application. Specialized custom made case spacers ('Stilty' etc) have a step cut inside and outside to precisely fit the movement. The long gone 'Stilty Spacers' were made to also support the dial using an eta 2836 or 2824 movement and are near impossible to find now.
It is easy to make a 28xx to rolex 1520/1570 spec case out of a standard brass spacer and anyone handy with a lathe can make a precision 'Stilty' type spacer using a standard brass spacer in an hour or so. The 'Yuki' spacers are also precision made as far as I know but I have never seen one.
https://yukiwatch.ecwid.com/ETA-adaptor-retro-fit-cal-15X0-case-p35063689
Maybe someone on the forum will make one for you.