You are correct. In CK find DB11LargeUrn (4FDAE) in WorldObjects>Container>CLUTTER>Ruins (it is enough to search in WorldObjects, there is but one such urn). Open it and in Item list you will see only one ObjectID (Gold001) with value 20000. Select it and under that window you have "Count" so change it to your liking.
But there is a catch. That value is 16bit signed integer. So you probably know what that mean. Maximum you can put there in CK is 65535 and anything over 32767 would probably cause problems. Unfortunately, NetImmersion engine is obsolete crap and this 16bit signed integer for common values is typical example of this crappines. This is why such simple, but logical request was not fullfilled. It will need either some papyrus scripting magic to increase that value significantly or even thinking out of box (like creating some super expensive jewlery which will not again overcome max gold value for merchants).
EDIT: You know what? That thinking out of box is not so bad idea. You can add to these money also some gems. For example DLC2TGGemSapphire is gem from another Dragonborn quest and has value 5000 for 0.2 weight, this means you can put in that urn 40 these gems and you will have 200,000 septims worth reward. With merchant gold modifier mod not a problem to sell them in larger cities. Or you can put there GemDiamondFlawless, that is more lore friendly, but has value only 1000 so for ame reward, you will need add 5 time more gems than DLC2TGGemSapphire.
Bet there is good news, you can have up to 32767 items in container (urn) so even with those diamonds, you can have 32 million worth reward. Only selling them to vendors will be for hundreds of gameplay
EDIT2: Well, I tried on my Aniversary instalation with mods and Unoficial pack and first adding max value (65525) of gold did not caused any problems. So obviously Bughesda made some changes in its engine. But secondly, I discovered, that there is no practical limit, how many times you can add Gold001. It looks like multiple items, but total value is sum of all Gold001 values. So For example, I added 3 times 65525. This resulted in game that in urn were three coins items with value 196575. I could pick any of them and in my purse was added exactly 196575 and all three gold items vanished from urn. So yeah, you can probably put millions in that urn doing that. It just broke little immersion, but otherwise, it worked flawlessly.
