I would measure the inside diameter of the pipe used for the verticals
and buy pipe the size that will fit inside it.

If you have a pipe bender make a U shape to connect the uprights.

Allow at least one foot overlap and drill and bolt the pipes together.

You can get steel straps made for timber frame construction to strap
the roof to the uprights.

On a sloped roof with a wire wall you need to tie the roof down so that
wind gusts under the roof do not lift it off.

If the kennels are not anchored securely to the ground I would not try
it at all.