10-20 lbs sounds like a lot of weight for tendrels to hold up, especially if you get some wind to move things around a bit. I have metal conduit pipe along the top of my fence trellis so the wire won't. I could probably hang some sling material from the pipe to hold up heavy fruit. Something to consider.

It's doable.. what I would suggest is using some sort of expanding netting to actually support the fruits as they ripen. Think using cut up pantyhose or something to that effect and then tying the top of the net/pantyhose to the trellis. It takes the weight off of the stem.

There's probably a good way to do this, but I have to admit that none of the pictures I've seen look very convincing. It seems hard to create a sling that will hold a full-size fruit and won't injure the vines. Some people use mesh bags, but you'd have to adjust it constantly as the fruit grows. Others set up platforms to support the fruit from below, but those have legs that have to be anchored in the ground near the vines, or even let the fruit drop from an overhead trellis.

One approach may be to build two sturdy trellises a few feet apart, with crossbars that can be inserted to support fruits. Kind of like a jungle gym for pumpkins.

I used panty hose last year and it worked just fine; looked a tad odd, but worked fine. The nice thing about the hose is that they expand as the fruit does. If the fruit gets too big for one hose, add on another.

Nice! Like this?