Ok what I'm about to tell you will work for BP's and this is what I used to do to keep my Anaconda's humidity 70%+ and that's with a overhead heat emitter. For this to work you need an UTH and you should NOT do this if your snake burrows or digs up any kind of layer. I'm only slightly familiar with reptibark and from what I know when I used to keep scorpions is that it doesn't hold humidity well, or at least it never did for me. Your Eco earth is fine, though I don't know how it'll affect what I'm going to tell you, when I did this I used the Forest Floor by Zoo Med which is cypress mulch. So you may or may not have to do away with the Eco Earth.
What you do is you want ideally about 3-4" of substrate for this to work well. You can use a straw or a PVC pipe about a dime size in diameter. You want two, one for each side, and you put them in the corner doesn't matter if the front side or rear side. But you need them to go to the bottom, but before you put them there you want to have an angled cut like "\" but more of an angle. Then what you do is you put it in the corner and then put the bedding around it(this is why I'm not sure if Eco Earth will work because it may just clog it, but then again it may work better than expected) and then you put enough substrate to cover up to the top of the PVC pipe and then from there you can just pour some water down the tube and the UTH will start to evaporate it and in turn it'll raise your humidity and it should raise it by 15% if not more, depending on how you set it up. I had an UTH on both sides so that way it evaporated quicker to raise the humidity even higher and in turn I poured a little more water. I think I kept her in a 65 gallon long and if I'm not mistaken I poured probably about 5-6 cups every 2-3 days depending on how it looked and I mean this in the strictest way possible that you want to check closely to make sure you're not getting any kind of mold or anything, which I never did but I always checked thoroughly, and every other week I rotated the substrate out so I could check on the very bottom and that way the substrate that stayed damp/wet had a chance to breathe. I also would move something over both PCV pipes so that way you're snake isn't tempted to stick it's head down there and because yours is so small you may want to do even smaller then that, something less than the width of it's mouth. Please just keep a close eye until you situate exactly how much water and the temps and everything, also the added UTH to the other side should help raise the ambient temps and if you still need to raise them more you can get the heat tape tape and put it along the sides of the substrate which will help raise your ambient temperature as well.
Now I'm sure some people will comment on this but I've done it and it worked fine.