WW I have done this on a show I was filming and we used Rinkhals, Puffadder and Snouted cobra venoms. They all caused the blood to remain liquid after 20 minutes in a clean glass vial and the control blood with no venom added had coagulated.
Possibly my "testing" was just a fluke and the blood donor might have taken some anticoagulant (asprin?) before the time, but his control blood did coagulate. You should still have some puffadder venom left over from what I sent you. Try placing this with 5ml of blood in a clean glass vial and see what happens after 20 minutes at room temperature. If you could do with some Echis venom this would also be interesting. Make sure the donor is not a haemophiliac or on blood thinners either.
I have seen some australian venoms cause a jelly blob in a test tube.