Essentialy you have created a homemade device similar to “The Grip.” I used the grip for a couple of months and gained from it - the max weight I applied was around 10lbs, mostly BTC. I would have liked to go higher in weight, but found the grip was unable to hold a good vacuum seal above 10lbs - at least for a suitable length of time.
I like the idea of stretching from the head, as opposed to clamping onto the shaft and stretching from there, but, perhaps influenced by my experience with the grip, I don’t think a vacuum device is the way to go. I had an edema reaction on my glans, and still have a small spot of discoloration at that location, further the circulation to the glans was compromised at higher weights. Despite these issues, I thought the device was serviceable, just not clearly better than a clamping technique in regards to penis health and safety. The nail in the coffin for the grip was that the malleable portions wore out fairly quickly - rendering the device unusable. Hopefully, you have overcome some of the limitations I found in the grip.