I got some velcro wrap things.. I loosened the fixture, stuck the end of the velcro wrap strips up under the edge of the fixture (two at each end) and tightened the fixture back up to hold the strips.. They hold the plastic up and actually don't look bad
8 in. x 1/2 in. VELCRO Reusable Ties (50-Pack) - 90924 at The Home Depot
One-Wrap 30 Ft. x 1-1/2 in. Strap - 91372 at The Home Depot
Those are two examples of the kind of product I use (I use the 1st one but the 2nd one might be better)
Additional Off Topic but related item:
One of my fixtures the bulbs wore out (heck it's been over 5 years) I replaced the Thin-Light lamps with a couple of Generics I got at Wall Mart.. The size code FT-xx_) was the same but the color temp was way higher.. Wattage is the same... So long as the size code and wattage are identical they should work ... And work they do.
The higher color temp means it's a lot whiter light, much nicer in my opinion. More "Natural"