Just find out what the torque is on that rear axle bolt, loosen it, turn both adjusters equally and torque down axle. It's too easy to need someone for.
Cut-in driveway or ditch trick. With no design or fabricating skills I wouldn't attempt to build anything. Very tough to make light enough and compact enough to fit in the bed with the bike if it's made out of wood. And very risky to try to use anything that isn't curved like that one you're seeing online.
Assuming you buy a fresh one, how long before you change it out (barring accidents) I try to get five years total out of it from date of manufacture. A lot of people think that's a little long.
He should've put a reserve on it. You may be out of luck in both the purchase and the bad feedback. If he's using 'broken extra face shield' as the excuse, then call him out and say, it's OK just sell it to me for less.
It's really encouraging that you accept your child's crows-foot defect so well! New shoes are always a trying time with kids. You're a real stand-up guy (no pun intended)