Before you mount it. take it outside and simply set it on the dash in the sunlight and make sure you can read the thing and it wont shut down from overheat in 5 minutes.
Hydraulic fittings do not need any sealant (NPT fittings are the exception, however they are poor choice of a fitting)
Your hydraulic cylinders use SAE inverted flare fittings
generally norcold is used in RV's and dometic was more in the marine market, however the lines are blurred lately. both make a reasonable product. Dometic has better customer service. Norcold's customer service is non-existent.
I woulnt use seacast. however 1/4 or 1/2 ply laminated would be a good start.
once you remove the cap you can get eyes on what was done at the factory.
local fuel dock had to have fire dept foam a customers boat because he put 20 gallons down a rod holder into the bilge. the fuel dock manager saw it and hit the e-stop