A mattress protector is a must, given that your Spindle’s cover is not washable.
A mattress protector goes over the quilted mattress cover before you put on your sheets and other bedding. Waterproof and washable is the way to go! The goal is to keep all moisture from reaching the mattress. A mattress protector preserves your Spindle, shielding it from the little accidents of life. We sweat a lot at night, so you want to make sure that you get a washable cover that will protect your mattress, reduce allergens and keep your bed fresh and clean.
Nearly all protectors we've seen are washable and that can help reduce allergens and keep the bed fresh. Whatever you put on top of the mattress will affect the feel, not necessarily in a "good" or "bad" way, just different. A good rule of thumb is the more you put on top of the mattress, the more it will affect the feel. The tighter the cover the firmer the mattress. The experience is subjective and everyone is different.
Mattress protectors come in different styles, e.g.: fully enclosed dust mite cover; waterproof fitted sheet style; down featherbeds; wool; simple cotton polyester pads; and many more. These are generally commodity products that try to differentiate themselves with fancy sounding titles and buzzwords, e.g.: "bamboo" [aka rayon]; natural; organic; pure; etc. Any style should do a good job. Just go with something you like and don't overthink it.
Today’s waterproof mattress covers aren’t all plasticky and crunchy like the waterproof of yesteryear. Most often waterproof mattress covers have a terrycloth top with a waterproof backing. This keeps you protected and you’d never guess it was waterproof! The slight downside to this one is that it sleeps a little warmer, but it really is the best at preventing moisture from getting to the mattress.
We think that these are great products at a very attractive price point. Searching shopping sites like Amazon will return other products with different features that get good reviews. You'll probably do well with most of the popular items, e.g.: 16,000 5-star reviews.
At the end of the day, the less expensive ones from Amazon or TJMax work great at protecting your bed. If you want wool to regulate temperature, you'll pay a premium, but sacrifice on the moisture protection.