Munchie,
Look at the following link for a straight leg raise exercise YouTube - Straight Leg Raise

In your case, forget about the extra weight, hold your leg for 2-3 seconds and perform at least 8-10 repetitions. You can try it with your toes looking at the outside (for vastus medialis) or upwards. This is supine. In prone position (with your front body facing the bed), try the same for hip extensors (gluteus, hamstrings) as well as back extensors. The other leg lies straight, it can't be bent in this position as in supine.

Concerning stretching, be careful at the following: 'stretching' can be achieved only by a constant elongation for at least 20-30 minutes. When applying the stretching exercise for the hamstrings you described, you 'relax' the muscle. That is different from stretching. If you relax the muscle, then the muscle pumping mechanism you want to improve for helping arterial and venous blood flow is less activated, because a relaxed muscle has a lower muscle tone at that time (and you want to increase it to help blood going back to the heart through the veins). On the other hand, that would be the case for exercise, to improve pumping mechanism. Since I'm not aware of effectiveness of stretching in DVT, I would suggest the same as for exercise, that is test and try.

You will find many others having their own opinion just because there is no specific evidence. So, your body will be your guide.

ilias