This publication by Greater Sydney Local Land Services has designs for arboreal mammals and small birds, including for Microbats: https://www.wires.org.au/wildlife-info/wildlife-factsheets/Wildlife-Nest-Boxes-LLS.pdf
The East Gippsland & Mafra and District Landcare Networks have produced a wonderful booklet with numerous nest box designs for birds and mammals, including notes on preferred food sources, habitat requirements and more for each included species. Below is their nest box design for Microbats, however please refer to the booklet for the complete instructions. Follow this link to access the entire booklet: https://egln.org.au/wp-content/uploads/2013/07/Nest-box-booklet.pdf
