Does anyone know where to get custom made slide covers? Or can I make them myself? I'm planning to get them for my section this year. I've only found one website that sells them.

You should be able to make them yourself. Depending on how well you sew and how complicated you wish to make them. If you don't sew find a friend who sews or has a parent that sews and have them teach you.

It will be fun and rewarding.

are the slide covers held together by velcro or just fitted exactly to the size of the slide?

Back when I was in school we had to lace them up. Each slide may be different. It would be easiest for you to attach velcro so it could be part way adjustable for different width slides.

So basically hem a large rectangle long enough for the length you want and able to wrap around your slide. Then stich on some velcro strips at the ends.

There is tubular foam insulation with a slit that will slip right over tubes. The fat edges glue themselves together upon contact. Home Depot surely has it in stock. You cut the length that you desire.