I would like to store some items before heading off to Spellhold since my inventory is packed with upgrade items that I can't use yet and was hoping someone could tell me where I can keep them without disappearing. I know I have seen this on this forum before but couldn't find it again, sorry.

There are many containers you can use. For example, you can put them in the chest in Copper Coronet (near Anomen's initial location).

All of the chests in your stronghold is safe after you get the stronghold.

I always keep my excess loot at the ground floor of the Shadow Thieves HQ. There's a ton of containers and it's nice to have the upgrade components in the same map as Cromwell.

That is a good place but when you done Renal's quest the map of the guildhouse is change and your items lost, after you done the quest it is safe, so be aware.

I prefer container in docks, between Maevar's house and Demi-lich's house. It's close to Cromwell and you don't have to enter any building

There's no problems by using a barrel next to Cromwell for storage? It's an outdoor one.

When I leave for Brynnlaw will I have to take my stuff with me or can I leave it in the container unharmed?

No problem , I leave my items everywhere in Athkatla , except on the ground . The item on the ground will dispeared after 12 or 24 hours ( I can't remenber it clearly) .

So you can use any containers , no matter whether is outside or where the container is .

Thanks!