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Sealed or not, if they are wet cells ideally you want the plates to remain fully submerged in electrolyte all the time. How far you can heel and still do that, depends on how much electrolyte they were designed to be filled with. (Which is one reason to keep them topped up.)
With AGM or gel batteries, there's no slosh so heeling isn't a factor. With wet cells, you just have to do the best you can. Try to do your installation as level as you can, plates which are not submerged will take uneven wear and age unevenly.
Instead of adding in a solid spill tray, maybe you can add a liner to the compartment, made up from shower pan or roofing membrane generously gathered under/around them, with some baking soda in the bottom as an acid absorbent?