I deal with this sort of issue for work on an almost daily basis
the 1550 has a panel frame ring as others have mentionedwww.frontpanelexpress.com
is a website that will allow you to download a free autocad type program, design a panel to fit that frame ring, then order it (approx $100 typically)
Put the body of the radios inside the case, then put the remote head on top of the panel frame designed above. You can also have holes cut for fuse holders, on off switches, etc. You can end up with something pretty slick if you take the time and spend the money on it
As was mentioned, you could put a piece of MDF or other wood along the bottom, glue that down and then screw into it to maintain waterproofness. Or, you can just bolt through the bottom and glue over the screw bolts.
Also as was mentioned, the question is how happy you will be with something like this as opposed to just using the pelican case to transport the gear then take it out.
if anyone would like more information or pictures, email me at ric0123 at h0tmail dot com. I'm not on this forum much but I'll share as much information as I can