Slipping Seat Post / Seatpost slip

This is exactly what I did on my mountain bike beater when I couldn't be arsed to buy the right size post. I used a coke can which worked great.

Buy one can of larger and a small tub of assembly paste.

Make a single layer shim from a neatly cut rectangle of beer can material (one or less wrap around the post).

Lightly 'key' (sand the surface very lightly to increase friction) both the inside and outside of the the shim - also key the part of the post that is hidden in your frame - and you could even try a light keying of the inside of the seat tube.

Now get your assembly paste.

Put a thin (must be thin!) layer of assembly paste on both the inside and outside of the shim and assemble the shim/post/clamp to taste.

Cheaper beer cans (supermarket own brand) = thinner alloy, more expensive booze = thicker cans. By the sound of it thinner may be better.
(Do not use Kronenberg - steel not alloy, will rust - Do not use Stella - embossed can causes tolerance problems)