Give the panel a name which people will feel proud to be a part of, and make it clear that it is a small group of selected people who make up the panel. Imagine a panelist telling a friend; "I am a panelist in Acme's innovation panel, we are only 50 people who create new things together".
You could also reward the panelists after a while, by inviting them to events or giving them some kind of perk like having lunch with the CEO. As people joined the panel to contribute, this is often more powerful than discounts and monetary rewards.
As answers start rolling in, make sure that you are active and ask follow up questions by commenting on the panelists' answers. This is an excellent opportunity to dig deeper, to understand more about why the panelist answered what they did, and to ask how they would want it to work instead.
It can be hard to quickly act on new insights, but it is easy to show that you listen and learn. Panelists want to feel like their effort is valued, so take the time to tell panelists what you learned from the previous video question when you record a new one. And when you have created something based on input from the panel, make sure to let them know!
When panelists see that other panelists write good answers and put effort into explaining their opinions, they are more likely to do so as well. Try to pick people who have shown interest in contributing before, perhaps in social media or through customer service. You also need enough panelists, we often recommend 50-100 people.