It's pretty easy to do, but you do need to be able to handle a needle and thread. If you can do a button, you can do this. The harness will be adjustable.
From the fabric store, you need 2 1" plastic snap buckles, 2 1" plastic adjustment slides, 2 yds. of matching 1" polypropylene webbing, thread, and a large curved upholstery needle. And a reg. piece of chalk.
Sit your child up straight in the stroller, then use the chalk to outline his neck and shoulders. Measure down diagonally from the neck/shoulder junction to the front center of the seat (where the child's knees would be), then add 12". Cut two lengths of webbing that long. From the rest of the webbing, cut 2 pieces 4" long. (You'll probably have some left over, 2 yds. is generous.)
Pass the cut ends of the webbing lightly through a flame, until they become stiff; they won't fray now. (Don't hold them in the flame; they will catch fire! The point is to melt the cut end just a little bit.) Thread the female sections of the buckles onto the short webbing pieces, the ends should double back. With the webbing ends doubled, sew these assemblies, buckle pointing upward, onto the back side of the belt harness, about 2" out on either side from where it now fastens. Now the bottom section is finished.
Thread each long webbing strip around the middle bar of an adjuster slide, then sew the loop shut about 2" from the slide. Now you should have two pieces, each with a loop on the end that contains a slide. Next, thread the loose end of each piece through a male buckle section, then back up through the adjuster slide, so that if you hold the loose end, you will be holding a large loop that has 1/2 a buckle on the loop side, and that can be adjusted to make it smaller. Check the length of these assemblies and cut them shorter if they are way too long (remember that they adjust in length). Finally, fold under 1" of the cut end, then sew the end of each strap assembly, upside down with the buckle end draped over the back of the seat, firmly onto the back of the stroller pad, about 1 1/2" higher than the child's shoulder line, and about 2" out from where his neck will be. When the strap is sewn on and then pulled forward and down as it will be buckled, the adjustment slides should be facing out, so that the looped end does not show (check the alignment before you sew it down.)
Now, adjust the strap length and buckle in the child. Anti-Houdini retrofit completed. (BTW, the reason for not just making loops on the ends of the shoulder harness pieces is so that Houdini cannot slide them sideways to escape, and the length can be adjusted as Houdini grows.)