Scratch building can be very difficult, and depending on how close you get to the GW original then their Legal team may have some complaints about any site that hosts templates.
The cabin shouldn't be too difficult to work out, although you'd need some good support for the inside of the corners. I can imagine the legs being a lot more difficult to get right, though.
Having said that, back around issue 100-130 (before I started, but within the region of magazines I have) GW used to put templates for what Forge World now do (big guns that are mounted on Rhinos to make them something else) in White Dwarf. The quality of the models at the time wasn't as good, but it was much more hobbyist and 'pure' gamer than just buying everything.
If you're just wanting to have a Sentinel without paying the excessive prices then have you read our
GW on a budget article? The best solutions are online retailers with a discount or eBay shops
It also means that even if it is still quite expensive, at least someone other than GW is getting a cut