Windows 95 doesn’t support more than two hard drives, so you’re out of luck – no drive e with 2 GB.
But I think your other idea is good: use a 2GB Win95osr.img instead of only 512 MB, copy the installer there, then install the game to hdd-2gb.img, and delete the installer afterwards to free up space on your Win95osr volume again.
For easy file handling, I suggest you use UltraISO. You can easily copy files to hdd images with this tool.
And don’t expect too much. Diablo 2 is hardware hungry, it runs veeery slowly in mobile Dosbox, unless you have a very powerful phone (=rooted & tweaked Galaxy S7 or similar).