Thick downtube protection tape

Have you considered neoprene ?. Comes sticky backed and in a range of thicknesses. It might afford some protection by softening the sharp block rocks cause. Being semi soft, its going to do more than a simple film of plastic.

Does anyone have any idea what this might be?

All Mountain Style? It's a kind of 2 layer honeycomb..

@dyna-ti I hadn't, worth a look though. The tape I'm thinking of is probably similar density, maybe a bit denser, but has that semi-soft characteristic I'm after. Imagine heli-tape with a few mm of semi-hard gel behind it.

@frog, nope not that, thats 0.38mm thick whereas this stuff was about 2mm and softer.

I got the impression it was cut from a roll, e.g. some sort of generic 3M tape, rather than being anything frame specific

Dyedbro film is pretty thick. So is this
Actually, the description matches door sill tape. It definitely has a slightly spongy feel.

I appreciate this isn't answering your question, but 3m Scotch 2228 rubber mastic tape does the trick. Bare in mind that it permanently deforms if you strap anything on top of it.

I had this on for a year or two but have recently removed and replaced with a Nukeproof downtube protector which fits pretty well and looks better, albeit at the expense of coverage.

Make something from Kydex? There are tutorials on YouTube.

I plan to do this for my new frame.

Invisiframe have cut me the downtube parts out of their thicker film in the past.
I asked and they delivered.
I just swapped the downtube and BB shell pieces out for the regular kit pieces, perfect fit.

Plenty of good aftermarket downtube protectors out there. Quick google will find you something better than thick film.

Nukeproof Dissent downtube protector is a pretty universal shape.

Thanks folks.

I'm pretty sure this is what I'm after.
https://www.xcracer.com/shop/viewproduct.php?productid=402

Really good stuff. Think of a tough heli-tape with a dense gooey layer behind to dissipate energy from rock strikes.

All Mountain Style? It’s a kind of 2 layer honeycomb

Pretty damn tough it is. Have it on my carbon frame and its never shown any sign of weakness. BPW is the rockiest I get though.

I remember CRC selling some stuff in sheets about 2mm thick years ago, I haven't seen it since, moulded protectors seem to be the best option these days, some are pliable enough to suit other branded frames.
That thick stuff was called Effex or something along those lines and came in clear or carbon effect.