I had the same initial problem with tubes not standing upright due to "shaped" caps. I just found a few more pieces of caps and adapters that were large enough to glue together in a sort of cloverleaf pattern such that they formed a base and the tubes could drop down in and be held upright (nearly vertical). Since these can be made from white PVC, maybe easier for you to find. I didn't have access to any power tools at the time so I hand cut everything with a miter box and back saw.