How can I create a block cipher of a reduced size given a fixed length block cipher?
If I understand your problem, I think the best way would be to pad the plaintext bits to 128 bits (8 bytes) with zeros and then use a strong block cipher like AES (Rijndael). Of course, then your ciphertext will be 128 bits instead of the 54 (or whatever) you started with. You can trim these trailing zeros after decryption.  If you want the ciphertext to be the same length, you'll have to use a stream method, but which will not be as strong.