I was experimenting with an idea and /boot was put on the first 256MB of a 1GB flash drive.
Windows destroyed the whole scheme when trying to add an NTFS partition after it in the unallocated space.
Can I make a new /boot without having to reinstall? The rest of the partitions are untouched and in perfect working order.
Please note that it's not the Master Boot Record that was destroyed, but the entire /boot partition. So this problem cannot be solved just by reinstalling the GRUB2 boot loader.
IF all the windows install did was delete the partition before you stopped it you can try something like testdisk to see if it can find the original partition's on the drive.
If windows started writing data to the drive it is entirely likely that that partition was overwritten.
There are also tools like R-studio linux which may be able to recover the data as well but that may end up being more of a hassle then reinstalling the boot partition.
Run Boot-Repair, click "Advanced options", go to the "GRUB location" tab, tick the "Separate /boot partition" option, apply.