You might check with a few museums, not necessarily nautical but that would help. See how they are put together and materials used. Many photos on line as well.

The one I made used plexiglass that came in 4ft X 8ft sheets that the supplier cut to my specifications. I used regular wood for the joints and ends stained to walnut. Total cost was about 100 US$.

Building a "fish tank" and inverting it over an 8ft model... hum. One slip and... disaster. I did that for my LCM (see photo), only under 20in, so the inverted "fish tank" is OK for smaller models.

BTW, did you need to remove the model from the case occasionally (running on the pond...)?

Lew
Florida ⛱️, USA 🇺🇸

Look for someone in your area who builds aquariums. Get suggestions from them and build one. Turn it upside down and place it over the model.
I would prefer one with a wood frame. Use the same method.
Barry

Check out the post from LewZ about a month ago. Search for display case on this site...very helpful. RR

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