Original boxes work well if you still have them, but I could imagine one being a little creative with strong plate and/or picture stands to prop up a water blaster for display. I've either put boxes blasters on display, positioned blasters carefully on glass shelving for display, as well as displayed blasters that are able to stand by themselves (e.g. the Super Soaker Monster XL). I'd also imagine that some decent-sized acrylic stands could be used also to help prop things up securely. The main thing is to ensure that a blaster doesn't end up falling over and off the shelf that could end up damaging the blaster in the landing.
If I can still find ways to display water blasters, I won't accept the "lack o' space" excuse.
Sidenote: this is about displaying blasters, not displaying *ALL* of one's blasters!