They are a pain to make, but it's all possible if you want to use the 33 forearms. You take a MP5 tube and cut it, then weld in a piece of sectional stock from a donor tube. HK parts sells the actual tube section stock. Yes they are all the same diameters across calibers.
Cut the MP5 tube close to the transition where it goes to the finned tube. Measure a section of the donor finned tube and weld the whole thing together. The hard part is getting all those little fins to look decent, unless you want a weld bead running through it which makes it look a little hacked together. There is no support piece on the inside of an MP5 tube, if I understand that question correctly.
Let me measure the 33 tube length when I get a chance, just have to pull it out of the safe.
FYI HK never made these.