Get a piece of allthread, or a long axle, put two nuts on it and some big washers, and stick it in the drop outs with the nuts and washers on the inside.
Then turn the nuts out so it spreads the swingarm.
Try to make sure it spreads evenly, or keeps your chain line where you want it , etc.
And make sure the bolt doesn't squirt out and hit you in the face under tension.
You'll have to go wider than you need because it'll try to squeeze back to it's original shape.
Make sure the tubes don't kink,crack, etc.