I think abs should work as well as anything for filling the hole. One thing to consider though is how flexible the abs sheet is compared to the bumper. I know that the fiber-flex will adhere to ABS, they say it will work with TPO, but there are very specific methods to use. This requires the use of a plastic welder. I've been playing with my new welder and the fiberflex some, it will adhere to problem plastics (I've tried it with PP, that stuff sucks). So a welder, comes with a small supply of fiberflex, some reinforcing mesh, sander/grinder, Urethane Supply's instructions, some practice, and then you'll have what you need to make the structural work. Then maybe some filler to make it presentable, maybe see if that duramix found above (and mentioned here:
http://www.autobodysupply.net/articles.asp?id=135) will also adhere to ABS. Don't get too impatient, there are people who have more insight and information around, I know there's at least one around this forum, as I am
by no means an expert. I would talk to a couple auto-body experts if I were you. There are great information and experienced pros in here, but this is, after all, a carputer forum.
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