Instead of side reins, use long bungy cords. I connected them, with double-end snaps, to the 1) D-ring on the saddle or surcingle, 2) through snaffle bit ring, 3) clipped to a D-ring on the girth. You can put knots in the bungy to easily adjust the length.
It’s a very forgiving approach and can serve as just a reminder while you transition to the long-lining. And, it certainly won’t cause any panic or stress/tension.
PS: I would avoid doing circles and start with straight lines only until you have a few weeks of conditioning under your belt, and his girth. ;>)