I have hit in my upper thigh when I was in the midst of a bad addiction that left me getting really creative with vein seeking. I also used a good one that runs up the inner leg near the knee at beginning of thigh
Trouble with your "top of thigh" spot that you can "see" is its very likely superficial and easily blown. "Seeing" the vein isn't nearly as important as feeling for it--you should get that spongy bouncy feel when you push lightly. A lot of "top of thigh" veins just aren't big enough in diameter to do that, leaving them for those who get more desperate than you are currently
I would look at the hand of the casted arm. Also, how far up does cast go? There are upper arm veins easier to hit than top of thigh. If you insist on looking at lower extremities, I would first look at calves as there is a large vein that runs up from by the ankle bone up the inside of your leg. That's the same one you can also access by the above the knee spot as I mentioned.
There are many spots people don't realize , but like I said the biggest thing when searching for a new one is the feel of it, not what you can see. As the visible ones are often very flat and superficial and blow right out.