Be wary of adding any letters that you don’t like the look of. If you feel it doesn’t have enough in it or it has too much, you will actually feel less inclined to memorize that alphabet. Make sure that each letter brings satisfaction, or at least doesn’t make you annoyed.
If your alphabet doesn’t sound the same as the English alphabet, you can still use English letters for reference, but create different types of noises that each of your made-up letters represent. You can create your own language with this fictional alphabet.
