I don't own a blood (yet), although building enclosures is fairly universal and only minor changes may need to be made to the design depending on the specie's indivual needs.
Here' is an enclosure I built for a BCI boa of mine, it measures 42" x 24" x 24" (l x w x h):
I simply used Oak plywood for the build, trimmed out for aethetics and stained, etc. I installed glass doors that slide with a lock, although hinged doors and acrylic/plexi makes for a much easier and much more cost effective build. Melamine is another popular choice for building... Ventilation is made through two vents/registers in the back and Heat Tape attached to a dimmer (rheostat) control temperatures. There are a number of websites with instructions for building custom vivs, I can try to dig some up that I used if you'd like. Please excuse the cord coming through the front and the makeshift lock, this was temporary and the photo is outdated lol...
To tailor an enclosure like this to a blood python, you'd likely decrease the height and may increase floor space slightly. Aspen makes for a fine substrate, although I know of others who use cypress mulch, coir, newspaper, etc. As I'm sure you know, bloods are fairly placid animals... they don't need anything too fancy to make them happy = )
Here is a picture I saved from a year or two ago that belongs to GiantKeeper Reptiles at
http://brongersmai.com . He was selling these caging units and I saved the photo:
Of course, these are Bloods (Python brongersmai) but care is similar if not identical to that of your Black sumatran STP from my understanding. These are four foot cages, although I think you could get by with three footers (correct me if I'm wrong, blood keepers!). This style of caging is readily available from a number of suppliers - typically made of either Plastic or PVC.
Hope this helps some - wish I had a nice viv setup for a blood but I don't own one yet. Again, the second photo does NOT belong to me, it belongs to GiantKeeper Reptiles as linked above.