That Manta glider is darned small, and any adjustments you make had better be smaller
Now, the 'tape elevon' trick is something that's probably the better bet here, as it can easily be adjusted between flights.
If the glider is consistently 'falling off' to one side during tosses, a *tad* of weight on the 'high' side wings tip can help. Yeah, clunky, but works in a pinch.
I've done PG's like the foamy Manta with plain old index cards, cut into a delta shape, with their outer tips canted up a bit (built in elevons). These are also easily flown, and mounted with a dowel for the 'body spine', extending some distance forward of the delta wing. This is for the necessary nose weight you'd need anyways. At the front of this, you can tape a small bent open (or cut in halfish) paper clip. Hook that on a forward lug, and let er rip.
This reminds me of the time back nigh 20 years ago, on RMR, where a guy was wanting *something, anything* easy to build gliders for a class. So, I sent him a cardstock type, built and folded, and stuffed into a plain envelope. Mailed it to him, and he flew the heck outta that. Don't recall if he ever did the class deal, though.
*THAT* then reminds me (excuse the rambling, getting all nostalgic here) of the time right after that, back around '97. A few guys flying near Orlando, at the old Red Bug site (which Roger darn well knows of, before it got developed), contacted me about flying with them.
OK, great, especially since they'd heard about the infamous AstronMike and his flying circus.
Took one of my old Marauder glider kits, parts all in the bag, the kind that required NO GLUE to construct. Proceeded to build this onsite in 10 minutes, flew it, and it outflew my own Marauder! BTW, the girl there flew the 1284 Shuttle a few times on C5-3s. As you'd expect, it tended to lay over on boost often, but one flight, it decided to go straight, and probably turned in the best 1284 flight in history.
Darn, I miss those days.....sorry for the off topic drifting here.