Saving bytes in custom data type

Hello,

I'm writing a variable size custom datatype in C. The variable part is
an array of unsigned long, and it needs to be aligned. I further need
to store a few flags, for which a single byte would be more than
enough (I would actually need just a single bit, but I'd probably keep
some bits to store a data version too).

Using a struct like

    {
        char vl_len_[4];         /* varlena header */
        unsigned char flags;
        unsigned long data[1];
    }

3 bytes are always wasted in padding as offsetof(data) is 8.

I may complicate fetching a little bit and store the flags at the end
of the data, so that the total size would be 5 + data instead of 8 +
data, and access them with some pointers arithmetic.

In terms of disk space, does it worth the hassle or (as I suspect)
would this effort be wasted by on-disk alignment of the data in the
rows?

Thanks,

-- Daniele