The blanking plate (part HYB3917 for Australia and CZH351 UK) had a gasket (part HYB4076 for Australia and ALA4817 for the UK).
A search on the ALA part number might locate the item from a Mini parts suppliers. Minispares don't list it and Somerford show it as NLA -
https://www.somerfordmini.co.uk/catalog ... ds=ala4817Attachment:
PUB1052 page RL2.jpg
If you plan to make your own you can use the specification and dimensions from the engineering drawing. It shows one version of the material as Rubazote, a trade name for high density rubber cushioning.
Click on image to enlarge, and again to enlarge further.Attachment:
HYB4076 Gasket Base Plate Intsrument Panel wm.jpg