I don't understand why you are finding this so difficult. Here is my method and it is relatively pain free.

Mount that tire on a rim (no glue), inflate it, and leave it there for a month.

Remove tire.

Using 3M contact adhesive (buy a can, way less mess than a tube) and a disposable paint brush, paint both rim and tire.

Wait an hour or two for them to dry. Do not join them!

Carefully mount one end of the tire on the rim, and simply roll the rest of it on the rim. You only get one shot to position it, so you must take great care, as once the two surfaces meet, they join instantly.

The glue allows you to position the tire wall perfectly against the rim with no fear of premount since the adhesive will not stick to anything other than another surface prepared with the same adhesive.

It is not tough, and it works great.

You can also use an old tubie prepped this way as a spare. The contact adhesive won't be so strong that it'll glue the tire together while it is stored, but it'll have enough to keep your tire on till you get back home.

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Elivis needs boats.