Create a block device in RAM

Just use brd and create one brd device (ram0). Use that device in place of your usb drive. You can partition it using sfdisk, use the partitions and then use dd to dump its contents to file.

There is no need to use one filesystem per brd device.

Or (though little hacky) you can use tmpfs, create image file and use that as loop device. That might be the easiest way to accomplish what you want. As a bonus, you have that image ready and can upload it straight away. No need to dd.

# Create mountpoint for tmpfs
mkdir /tmp/tmpfs
# Mount tmpfs there
mount -t tmpfs none /tmp/tmpfs
# Create empty file of 600MB 
# (it creates 599MB hole, so it does not 
#  consume more memory than needed)
dd if=/dev/zero of=/tmp/tmpfs/img.bin bs=1M seek=599 count=1
# Partition the image file
cfdisk /tmp/tmpfs/img.bin 
# Create loop block device of it (-P makes kernel look for partitions)
losetup -P /dev/loop0 /tmp/tmpfs/img.bin 
# Create filesystems
mkfs.vfat /dev/loop0p1 
mkfs.ext4 /dev/loop0p2
# Now it's your turn:
#   mount loop0p1 and loop0p2 and copy whatever you want and unmount it
# detach the loop device
losetup -d /dev/loop0
# May i present you with your image ... 
ls -al /tmp/tmpfs/img.bin

Modify to suit your needs.