I am actually serious about testing this out in-cave though (NOT in Ellison's the first time mind you! haha). The washers edges are actually smoothed out some and the groove they create is rather deep. Only one eyelet has a gap (a small one) which was cut open, the other is fully closed and the webbing can't come off it it. The gap in the other is so the roller can be taken on and off the chest. If you make sure to hook it so the gap is away from your chest, it is actually pretty secure. I've tested it on rope in tree and it works as intended. Not all that comfortable and you have to pay attention to keep the rope running smoothly through it (don't lift your arms too much), but if you don't it isn't a big deal. The rope stays in the roller groove and the webbing stays hooked securely.
Another very key detail I "forgot" to mention is the fact that a safety ascender about head high and linked to your harness via cowstail would be mandatory with this roller, and also that it would likely NOT be useful for a real ropewalker. If it fails with it augmenting a frog or texas then you are just on frog or texas; not that big of a deal. A chest ascender could be used as the safety (such as if you were augmenting a frog system), but it would be awkward to use and might interfere with your knee height ascender and so it would be a good idea to have an extra hand ascender to clip in above you and then unclip your chest ascender when using the roller. For texas, the need for the extra ascender for a safety isn't an issue as your hand ascender is already rigged for being the safety like you'd want.
I also hoped that the shitty looking homemade nature of it scares people, haha.