1) THE STAMP: 

The stamp can depict whatever you deem to be appropriate for the Letterbox. Usually, it will relate in some manner to the location in which it is placed; other stamps may commemorate a person or an event. The choice is yours.

Stamps can be commercially made or hand made. Commercially-made stamps can be replaced if they are lost (if you kept your artwork), and can support MUCH finer detail, especially text. Hand-made stamps have a different quality to the image they make. Which to use is a personal and artistic choice.

Commercial stamps are of vulcanized red-rubber made from your artwork.  Expect to pay in the range of $10 for a 2" x 3" stamp.

Hand-made stamps are usually carved, either from carving medium, or from erasers (white vinyl usually, some use red rubber erasers).  Red rubber is reported to withstand extreme heat (Texas-style) better than vinyl. I don't know yet the effects of winter's cold.

Sources for, and critique of, carving media can be found within the Carving Consortium web page.  Most art supply stores usually only carry "Speedy-Cut" (shown in the previous photo), a most inferior, crumbly product.  Try instead using whatever white vinyl erasers they carry. Look carefully for surface defects such as logos, bubbles, pockets, and extrusion lines - these all will show up when you stamp the image. We use one called "Factis", measuring 1.5 x 2.75 inches, thick enough to be carved on both sides if you choose (under $2). It needs no backer block, unlike thinner materials. Also check out any other place selling office or drafting supplies. Lots of options exist. Size is also your choice - larger sizes are easier to carve (bigger stuff), harder to stamp well. 2" x 3" is a nice size, but trial and error will tell you what's right for you.