Building a DER sequence

[Salz, Rich]

    Do I construct a sequence and add items to it - top down?

No, because then you have to go back and patch the sequence length and perhaps slide everything up or down a copule of bytes.

I would look at an existing simple sequence and start writing your own based on that; look for ASN1_SEQUENCE macros in crypto/x509/x*.c files.  Another set of macros will declare the i2d/d2i and PEM functions if needed.