I (Martin) have build many of those drum rafts in my life. I was part of "Voortrekkers", like boy and girl scouts, in my younger days and we had what we called "water camps" every June holidays (mid winter here), where we built and sailed/rowed these rafts on a dam. Also rafted down river in these rafts....and sometimes in truck tire tubes too, for week long passages.
We also had a few Junk-Raft designs, but 2de hand plastic drums in South-Africa cost about $25 each these days. The design called for more than 50 drums, 3 JoJo tanks (large 2600l water tanks)to be used as cabins, 60m + of timber and steel, and miles and miles of line. In the end cost came into play, and truth is that Ply over frame would cost just a bit more than the Junk Raft. Nothing is free in South-Africa
I have seen Floating Neutrinos. Also looked at PlasTiki and Junk too. All of these has given us much of our inspiration. They are proof that on a low budget like ours, we can follow our dream. They may not have the looks, they may not sail the best, they are all risky to travel upon, but you use what you have.
On a side note, our move is done, just unpacking things, which is a bit of a nightmare going from 2 bed room apartment to just a room. We will soon start the planning phase of our 1/3 model/dingy. No rush as there aren't much funds free at this time.