I have a Lagoon 25. It fits snugly but most importantly doesn't completely cover the chamber. You need some slack in case the filter gets clogged. I currently use a sponge filter with a thin buffing pad and keep a seawater block in the chamber under the mount. It's relatively expensive considering it's a small molded piece of acrylic, but it's a fairly high quality niche product and suits my needs.
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