i'm kinda with you. on the one hand, the deceased have gone onto whatever the next great adventure is, and i doubt they care. but i do think that respect for the dead is important, and if the ashes are going to be in limbo for a while, i think it would be a lovely gesture to place them in a beautiful receptacle of some sort.
my BIL's ashes are in that sort of limbo. my parents-in-law haven't been able to bear to have them interred yet (they're going to have him placed in their burial plot with them since he didn't have any arrangements made.) for the nonce he's on their living room shelf, in a beautiful box that my husband made specifically to hold his remains.
i don't know what the crematorium put your in-laws in, but i know that both my BIL and my grandmother-in-law were delivered in plastic bags. definitely not an appropriate long-term storage plan.
i think it's very nice that you are putting such consideration into this.
ETA like feline, my dh and i have decided we want to be scattered together, in a particular spot. so whomever goes first will be 'held' until the other joins him or her. but we haven't discussed what to do with the first set- that'll be up to the last one to go.
khairete
S.