writer.py

with open('modtext.txt','rb') as rp:
    with open('make.bin','wb') as wp:
        sumlen=0
        head=sumlen.to_bytes(4,'little')
        body=b''
        for line in rp:
            sumlen += len(line)
            head += sumlen.to_bytes(4,'little')
            body+=line[:-1] + b'\0'

        wp.write(head)
        wp.write(body)