This is a very simple do as you've already done most of the work.
All you need now, is a Partition Manager that support formatting HFS. Google for it or start with this list:
https://en.wikipedia.org/wiki/List_of_disk_partitioning_software
Some things off the top of my head:
MacDrive and TransMac. Both have limited time evaluation trials.
Live CD of GParted, has HFS support.
And there are several other Partition/ Disk Management tools out there that will do it.
PS: It may also be possible to do it via the native disk/ partition tools on the OS, but probably harder.