Following a pretty bad attempt at making one of these myself, I decided to invest in something a bit better!
This does the job perfectly, I read some reviews on other harp holders, and many people have said the screws do not tighten sufficiently or become loose, I have not had this problem with this holder. The arms do not fall down, even with vigorous playing.
The springs hold the harp in position firmly, with no movement, and the rubber grips add to the stability, whilst giving you peace of mind without worrying about scratching or just general damaging.
I would recommend this product over any others about, it is low cost, good quality, and does everything that it needs to do!