I picked up one of the Walgreen's Pill Fob cases today. (Found just one on clearance for $1.99 vs. $4.99 regularly - are they dropping them?).
It will hold 1 or 2 of my 808 cartos without condoms, only 1 with condom. (Don't know how 808's compare to other sizes.) This is fine for settling *in* a juice. However, it is not very tall: if you want to reverse spin to remove a juice, screwing down the fob's cap will leave only about 1/4 the length of the condom to collect expelled juice in.
Assuming 501 cartos are shorter and narrower than 808s (I have no idea myself) this may work better for 501s than 808s.