It was in the sixties that I first took an interest in folk harps and
tried to find one to play all to no avail.  Ironically it was the very
time of the renaissance of the folk harp although I had no way of
knowing it at the time.  Years later I noticed a small harp gathering
dust on a friend's mantle.  It had been made for her by a local
dulcimer maker who herself had in all likelihood never seen a harp.  
My friend was into fantasy stories and the SCA and the harp had
been designed along the lines of what one imagined a harp of
mythic proportions must have looked like than for any acoustical
considerations.  The harp had originally been strung with banjo
strings and by the time I had seen it, several of those strings had
been replaced by nylon guitar strings.  Some of the strings,
especially the middle C string, had a very dull sound to them. The
original wound steel string had been replaced with a wound nylon
string to improve the tone but to little avail.  
I set out to find what type and gauge of string would give a good clear sound for that note.  In my ignorance
I made the intuitive mistake that if the string sounded dull, it was too loose.  So if I replaced it with a thicker
string, the string would need to be tighter in order to be up to that same pitch  To my surprise, it sounded
exactly like the thinner string!  A dozen different gauges of nylon and steel, wound and monfilament strings
were substituted and all sounded terrible.

It was after much study that I understood that for any given string material and a given note, there is one
length at which the string will sound with a clear tone.  Making the string thicker or thinner does not affect
the tone.  For that harp there was no string that could be put in that space and  be tuned to middle C and have
a good tone.
In practical terms this means that harp cannot be made to a fanciful size and shape and then afterwards fitted
with strings.  It is just the opposite.  First the range and tonal quality of the strings must be decided, then the
string lengths calculated, and then the wood must be designed around the strings.
In the long time from that harp to the ones I'm making today, here are the criteria that have evolved for the
ideal folk harp:
The harp must sound good and be comfortable and easy to play.  This is tautological and hardly
needs to be listed.
It must have enough range to play folk music and just a few notes below that for counter melody,
accompaniment, and resolving chords and such.
It must be portable, small enough to carry around, carry outdoors, and not require a specialized
vehicle in which to haul it.
It should be as mechanically simple as possible and also sturdy enough that it isn't in danger
of flying apart if it is in the least banged about.
The strings need to be of some common and easily obtained material, not rare, expensive, and
custom made for that particular harp.
It should have a natural finish that is easily obtainable and easy for the player to renew.
This is the big one.  It should not cost as much as the player's car or house, or car and house
combined.  The player should not be in a constant panic at every gig and session that something is
about to  happen to the most expensive thing they own.  The harp should be a source of fun and
relaxation, not panic.  So the harp must be affordable if not down right cheap.