how can i put the "remaining length"-element not at the end of a structure, but between fixed-length-elements?

Question

i have a fixed-length-chunk at the end of every section of my file,
but i don't know the length of the variable-length-binary.
how can i put the variable size between fixed-length-elements?
it only works if the remaining length element is at the end.

the structure looks like this:
chunk_1 (fixed length)
    fixed length binary
    variable length binary (remaining)
    fixed length binary
chunk_2 (fixed length)
    fixed length binary
    variable length binary (remaining)
    fixed length binary
chunk_3 (fixed length)
    fixed length binary
    variable length binary (remaining)
    fixed length binary