He's already doing the fun part
designing and developing good planes is very enjoyable. Designing them so they are simple and easy for someone else to build is very challenging, but the great feeling when someone else successfully builds, flys and enjoys one of your designs is one of the best feelings there is.
Making kits and running a sales business is work. A lot of work. A lot of work means frustration and headaches. Some of us are in this hobby as a distraction from the high pressure lives we live at work and home. Turning our hobby into work would ruin it, for some of us.
I bet if someone approched Glen to get permission to kit one of his planes and just pay him a royalty / commission, he may leave the door open. But, since designs are easily copied, modified, washed and waxed until they become someone else's labor of love, most people just do it themselves... Some better than others, some very well.
So, if there's a kit in particular that you want, try working with one of the kit vendors (like Hippo) to see if they can cut you some foam.