Someone asked this question today and it is so good, I went out and took pictures of the carrier I use and will talk about it and two other methods.
PanelPak offers pre-made popular frame sizes. They will make up a special size for an additional cost - like my 6x6 and 8x16 sizes. I use them constantly. You can either put one of their false panels on one side and only bring one panel for painting on - OR - bring two panels and have them both face inward - so you can carry two wet paintings. The panels do not touch each other, and you rubberband them together. The panels fit snugly in the wooden frame and slide right into your backpack. The picture below shows how the rubberbands go around the complete package.
RayMar Wet Panel Carriers:
Some people use a small pizza box and use push-pins to secure the corners of a painting inside. This might work for walking short distances, but would not work for hiking because you would have to have to carry it by hand and carry it level so the painting doesn't flop onto the inside of the lid of the box.
There are other small wet panel carriers out on the market, but they are usually made of wood and can be heavy and take up a lot of room. The first two methods above are the best I have heard of so far.
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