For the same cost as having the correct tool delivered, I bought a selection of cheap tools to trial:
L-R: Strap wrenches, oil filter plier, oil filter loop wrench and waterpump pliers 12”. These are tools readers are more likely to already have in the home or garage, or available to borrow from a friend or neighbour. These ones came from a glamourised “dollar store” type of shop for cheapness but also available at DIY shops or automotive shops.
Firstly, the strap wrench and the waterpump plier are no good. The strap wrench slips off as the surface is too narrow, meanwhile even the large 12” pliers are not suited to the size of the dust cover and don’t grip sufficiently.
Both of the oil filter tools are about the right size to do the job and worth a try. The plier version works best.
The ring tool needs a little care to get a good grip and must be turned over the right way to tighten or loosen.
Like this to loosen clockwise:
For completeness, this is the orientation for tightening (spot the difference), if deemed necessary to use a tool at all:
Finally, the recommended tool is the oil filter pliers, seen here. They grip really well but you have fine control over the grip (cover is only plastic) and good leverage:
As I said, these are tools you might already have or can access easily for occasional jobs (proper tool is available from Shimano). Gentle force should be needed, maybe even by hand. In my case the cover was seized tightly and needed some extra help, now I have a couple of tools to do it and may find other uses for them, especially the pliers.