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A strap will do and so will a piece of rope and a stick to twist it tight. When the tire takes air drop the stick. Air pressure can also ruin a strap if you don't get it released in time.
Loop it around the tire and cinch it up, tight.
As you fill the tire with air, have your finger on the cam release. When the tire seats on the bead, push in the cam release. No damage to tire or strap.
If you still have trouble, soak a towel with water and stick it between the tire and bead on the bottom, foot on the rim, while pulling up on the tire to seat the top bead while someone blows air in.
when no strap ect available, bouncing the tire on the tread, and rolling then boucning it again and again will help spread the beads of the tire out to seat them.